Semiconductor device

ABSTRACT

A semiconductor device including a transistor having excellent electrical characteristics is provided. Alternatively, a semiconductor device having a high aperture ratio and including a capacitor capable of increasing capacitance is provided. The semiconductor device includes a gate electrode, an oxide semiconductor film overlapping the gate electrode, an oxide insulating film in contact with the oxide semiconductor film, a first oxygen barrier film between the gate electrode and the oxide semiconductor film, and a second oxygen barrier film in contact with the first oxygen barrier film. The oxide semiconductor film and the oxide insulating film are provided on an inner side of the first oxygen barrier film and the second oxygen barrier film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device provided with atransistor including an oxide semiconductor film and a manufacturingmethod thereof.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate (also referred to asthin film transistor (TFT)). Such a transistor is applied to a widerange of electronic devices such as an integrated circuit (IC) and animage display device (display device). A silicon-based semiconductormaterial is widely known as a material for a semiconductor thin filmapplicable to a transistor. As another material, an oxide semiconductorhas been attracting attention.

For example, a transistor including an oxide semiconductor containingindium (In), gallium (Ga), and zinc (Zn) as an active layer is disclosed(see Patent Document 1).

Moreover, a technique in which an oxide semiconductor layer having astacked-layer structure improves carrier mobility is disclosed (seePatent Documents 2 and 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2011-138934-   [Patent Document 3] Japanese Published Patent Application No.    2011-124360

SUMMARY OF THE INVENTION

One of defects included in an oxide semiconductor film is an oxygenvacancy. For example, the threshold voltage of a transistor including anoxide semiconductor film which contains oxygen vacancies in the filmeasily shifts in the negative direction, and such a transistor tends tohave normally-on characteristics. This is because electric charges aregenerated owing to oxygen vacancies in the oxide semiconductor film andthe resistance is thus reduced. The transistor having normally-oncharacteristics causes various problems in that malfunction is likely tobe caused when in operation and that power consumption is increased whennot in operation, for example. Further, there is a problem in that theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor over time or due to a stress test isincreased.

Then, in one embodiment of the present invention, a semiconductor deviceincluding a transistor having excellent electrical characteristics isprovided. Alternatively, a semiconductor device having a high apertureratio and including a capacitor capable of increasing charge capacity isprovided.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode, an oxide semiconductor film overlapping thegate electrode, an oxide insulating film in contact with the oxidesemiconductor film, a first oxygen barrier film provided between thegate electrode and the oxide semiconductor film, and a second oxygenbarrier film in contact with the first oxygen barrier film. The oxidesemiconductor film and the oxide insulating film are provided on aninner side of the first oxygen barrier film and the second oxygenbarrier film.

Another embodiment of the present invention is a semiconductor deviceincluding a first gate electrode, an oxide semiconductor filmoverlapping the first gate electrode, a first oxygen barrier filmprovided between the first gate electrode and the oxide semiconductorfilm, an oxide insulating film in contact with the oxide semiconductorfilm, a second oxygen barrier film in contact with the oxide insulatingfilm, and a second gate electrode overlapping the oxide semiconductorfilm with the oxide insulating film and the second oxygen barrier filmprovided therebetween. The first oxygen barrier film and the secondoxygen barrier film are in contact with each other. The oxidesemiconductor film and the oxide insulating film are provided on aninner side of the first oxygen barrier film and the second oxygenbarrier film. Moreover, a side surface of the oxide semiconductor filmfaces the second gate electrode.

The first gate electrode and the second gate electrode are connected toeach other in an opening in the first oxygen barrier film and the secondoxygen barrier film.

The oxide insulating film in contact with the oxide semiconductor filmmay include an oxide insulating film containing more oxygen than that inthe stoichiometric composition. The oxide insulating film containingmore oxygen than that in the stoichiometric composition is an oxideinsulating film of which the amount of released oxygen atoms is greaterthan or equal to 1.0×10¹⁸ atoms/cm³, or greater than or equal to3.0×10²⁰ atoms/cm³ in thermal desorption spectroscopy (TDS) analysis inwhich heat treatment is performed such that a temperature of a filmsurface is higher than or equal to 100° C. and lower than or equal to700° C. or higher than or equal to 100° C. and lower than or equal to500° C.

Further, a first conductive film in contact with the oxide semiconductorfilm may be included in the semiconductor device. The first conductivefilm serves as a pair of electrodes. In addition, a second conductivefilm in contact with the second oxygen barrier film and the firstconductive film may be included. The second conductive film serves as apixel electrode.

Further, in the semiconductor device, a capacitor may include a filmhaving conductivity over the first oxygen barrier film, the secondoxygen barrier film in contact with the film having conductivity, andthe second conductive film.

Note that the film having conductivity is a metal oxide film including ametal element contained in the oxide semiconductor film, and includes animpurity. As the impurity, hydrogen, boron, phosphorus, tin, antimony, arare gas element, alkali metal, alkaline earth metal, and the like aregiven.

The first oxygen barrier film and the second oxygen barrier film are incontact with each other with the oxide semiconductor film and the oxideinsulating film provided on an inner side of the first oxygen barrierfilm and the second oxygen barrier film. Therefore, the movement ofoxygen contained in the oxide insulating film to the outside of thefirst oxygen barrier film and the second oxygen barrier film can besuppressed. As a result, oxygen contained in the oxide insulating filmcan be moved to the oxide semiconductor film efficiently, and the amountof oxygen vacancies in the oxide semiconductor film can be reduced.

Further, in the transistor including the first gate electrode and thesecond gate electrode, the oxide insulating film, which is subjected toelement isolation, overlaps the oxide semiconductor film. Moreover, inthe cross-sectional view in the channel width direction, end portions ofthe oxide insulating film are positioned on an outer side than the oxidesemiconductor film, and a side surface of the oxide semiconductor filmfaces the first gate electrode or the second gate electrode. As aresult, due to the electric field of the first gate electrode or thesecond gate electrode, generation of parasitic channels at the endportions of the oxide insulating film are suppressed.

On an element substrate of one embodiment of the present invention, oneelectrode of the capacitor is formed at the same time as the oxidesemiconductor film of the transistor. In addition, the conductive filmthat serves as a pixel electrode is used as the other electrode of thecapacitor. Thus, a step of forming another conductive film is not neededto form the capacitor, and the number of steps of manufacturing thesemiconductor device can be reduced. Further, since the pair ofelectrodes has a light-transmitting property, the capacitor has alight-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

According to one embodiment of the present invention, a semiconductordevice including a transistor having excellent electricalcharacteristics can be provided. Alternatively, a semiconductor devicehaving a high aperture ratio and including a capacitor capable ofincreasing charge capacity can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a block diagram and circuit diagrams illustrating oneembodiment of a semiconductor device;

FIG. 2 is a top view illustrating one embodiment of a semiconductordevice;

FIG. 3 is a cross-sectional view illustrating one embodiment of asemiconductor device;

FIGS. 4A to 4D are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device;

FIGS. 5A to 5D are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device;

FIGS. 6A and 6B are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device;

FIGS. 7A and 7B are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device;

FIG. 8 is a top view illustrating one embodiment of a semiconductordevice;

FIG. 9 is a cross-sectional view illustrating one embodiment of asemiconductor device;

FIGS. 10A to 10C are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device;

FIGS. 11A and 11B are cross-sectional view each illustrating a structureof a transistor;

FIGS. 12A and 12B each show calculation results of current-voltagecurves;

FIGS. 13A and 13B show calculation results of potentials of atransistor;

FIGS. 14A and 14B illustrate a model;

FIGS. 15A to 15C illustrate a model;

FIGS. 16A to 16C each show calculation results of current-voltagecurves;

FIG. 17 is a cross-sectional view illustrating one embodiment of atransistor;

FIGS. 18A and 18B are cross-sectional views each illustrating oneembodiment of a semiconductor device;

FIGS. 19A to 19C each illustrate a band structure of a transistor;

FIG. 20 shows nanobeam electron diffraction patterns of an oxidesemiconductor;

FIGS. 21A to 21C are top views each illustrating one embodiment of asemiconductor device; and

FIGS. 22A and 22B are cross-sectionals views each illustrating oneembodiment of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described in detail below withreference to the accompanying drawings. However, the present inventionis not limited to the following description and it is easily understoodby those skilled in the art that the mode and details can be variouslychanged without departing from the scope and spirit of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. Inaddition, in the following embodiments, the same portions or portionshaving similar functions are denoted by the same reference numerals orthe same hatching patterns in different drawings, and descriptionthereof is not repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where an etching step is performedafter a photolithography process, a mask formed in the photolithographyprocess is removed after the etching step.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of thepresent invention is described with reference to drawings. Note that inthis embodiment, a semiconductor device of one embodiment of the presentinvention is described taking a display device as an example. Inaddition, an oxide semiconductor film is used as a semiconductor film inthis embodiment.

FIG. 1A illustrates an example of a semiconductor device. Thesemiconductor device in FIG. 1A includes a pixel portion 101, a scanline driver circuit 104, a signal line driver circuit 106, m scan lines107 which are arranged in parallel or substantially in parallel andwhose potentials are controlled by the scan line driver circuit 104, andn signal lines 109 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the signal line drivercircuit 106. Furthermore, the pixel portion 101 includes a plurality ofpixels 103 arranged in a matrix. In addition, capacitor lines 115arranged in parallel or substantially in parallel are provided along thesignal lines 109. Note that the capacitor lines 115 may be arranged inparallel or substantially in parallel along the scan lines 107. The scanline driver circuit 104 and the signal line driver circuit 106 arecollectively referred to as a driver circuit portion in some cases.

Each scan line 107 is electrically connected to the n pixels 103 in thecorresponding row among the pixels 103 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 103 in the corresponding column among the pixels 103arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 103 in the corresponding row among the pixels 103 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel along the signallines 109, each capacitor line 115 is electrically connected to the mpixels 103 in the corresponding column among the pixels 103 arranged inm rows and n columns.

FIGS. 1B and 1C each illustrate an example of circuit configurationsthat can be used for the pixels 103 in the display device illustrated inFIG. 1A.

The pixel 103 in FIG. 1B includes a liquid crystal element 121, atransistor 102, and a capacitor 105.

The potential of one of a pair of electrodes of the liquid crystalelement 121 is set in accordance with the specifications of the pixel103 as appropriate. The alignment state of the liquid crystal element121 depends on written data. A common potential may be supplied to oneof the pair of electrodes of the liquid crystal element 121 included ineach of a plurality of pixels 103. Further, the potential supplied tothe one of the pair of electrodes of the liquid crystal element 121 inthe pixel 103 in one row may be different from the potential supplied tothe one of the pair of electrodes of the liquid crystal element 121 inthe pixel 103 in another row.

The liquid crystal element 121 is an element that controls transmissionand non-transmission of light by an optical modulation action of liquidcrystal. Note that optical modulation action of a liquid crystal iscontrolled by an electric field applied to the liquid crystal (includinga horizontal electric field, a vertical electric field, and an obliqueelectric field). Examples of the liquid crystal element 121 are anematic liquid crystal, a cholesteric liquid crystal, a smectic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, aferroelectric liquid crystal, and an anti-ferroelectric liquid crystal.

As examples of a driving method of the display device including theliquid crystal element 121, any of the following modes can be given: aTN mode, a VA mode, an ASM (axially symmetric aligned micro-cell) mode,an OCB (optically compensated birefringence) mode, an MVA mode, a PVA(patterned vertical alignment) mode, an IPS mode, an FFS mode, a TBA(transverse bend alignment) mode, and the like. Note that the presentinvention is not limited to this, and various liquid crystal elementsand driving methods can be used as a liquid crystal element and adriving method thereof.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

In the structure of the pixel 103 described in FIG. 1B, one of a sourceelectrode and a drain electrode of the transistor 102 is electricallyconnected to a signal line 109, and the other is electrically connectedto the other of the pair of electrodes of the liquid crystal element121. A gate electrode of the transistor 102 is electrically connected toa scan line 107. The transistor 102 has a function of controllingwhether to write a data signal by being turned on or off.

In the structure of the pixel 103 illustrated in FIG. 1B, one of a pairof electrodes of the capacitor 105 is electrically connected to acapacitor line 115 to which a potential is supplied, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 121. The potential of the capacitor line 115 isset in accordance with the specifications of the pixel 103 asappropriate. The capacitor 105 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel 103 in FIG. 1B,the pixels 103 are sequentially selected row by row by the scan linedriver circuit 104, whereby the transistors 102 are turned on and a datasignal is written.

When the transistors 102 are turned off, the pixels 103 in which thedata has been written are brought into a holding state. This operationis sequentially performed row by row; thus, an image is displayed.

The pixel 103 in FIG. 1C includes a transistor 133 performing switchingof a display element, the transistor 102 controlling pixel driving, atransistor 135, the capacitor 105, and a light-emitting element 131.

One of a source electrode and a drain electrode of the transistor 133 iselectrically connected to the signal line 109 to which a data signal issupplied. Moreover, a gate electrode of the transistor 133 iselectrically connected to the scan line 107 to which a gate signal issupplied.

The transistor 133 has a function of controlling whether to write a datasignal by being turned on or off.

One of a source electrode and a drain electrode of the transistor 102 iselectrically connected to a wiring 137 serving as an anode line, and theother is electrically connected to one electrode of the light-emittingelement 131. The gate electrode of the transistor 102 is electricallyconnected to the other of the source and drain electrodes of thetransistor 133 and the one electrode of the capacitor 105.

The transistor 102 has a function of controlling current flowing throughthe light-emitting element 131 by being turned on or off.

One of a source electrode and a drain electrode of the transistor 135 isconnected to a wiring 139 to which a reference potential of data issupplied, and the other is electrically connected to the one electrodeof the light-emitting element 131 and the other electrode of thecapacitor 105. Moreover, a gate electrode of the transistor 135 iselectrically connected to the scan line 107 to which the gate signal issupplied.

The transistor 135 has a function of adjusting the current flowingthrough the light-emitting element 131. For example, when the internalresistance of the light-emitting element 131 increases because ofdeterioration or the like, the current flowing through thelight-emitting element 131 can be corrected by monitoring currentflowing through the wiring 139 to which the one of the source and drainelectrodes of the transistor 135 is connected. The potential supplied tothe wiring 139 can be set to 0 V, for example.

One of the pair of electrodes of the capacitor 105 is electricallyconnected to the gate electrode of the transistor 102 and the other ofthe source and drain electrodes of the transistor 133, and the other ofthe pair of electrodes of the capacitor 105 is electrically connected tothe other of the source and drain electrodes of the transistor 135 andthe one electrode of the light-emitting element 131.

In the structure of the pixel 103 in FIG. 1C, the capacitor 105functions as a storage capacitor for storing written data.

The one of the pair of electrodes of the light-emitting element 131 iselectrically connected to the other of the source and drain electrodesof the transistor 135, the other electrode of the capacitor 105, and theother of the source and drain electrodes of the transistor 102.Furthermore, the other of the pair of electrodes of the light-emittingelement 131 is electrically connected to a wiring 141 serving as acathode.

As the light-emitting element 131, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 131 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the wiring 137and the wiring 141, and a low power supply potential VSS is supplied tothe other. In the structure of FIG. 1C, the high power supply potentialVDD is supplied to the wiring 137, and the low power supply potentialVSS is supplied to the wiring 141.

In the display device including the pixel 103 in FIG. 1C, the pixels 103are sequentially selected row by row by the scan line driver circuit104, whereby the transistor 133 is turned on and a data signal iswritten.

When the transistor 133 is turned off, the pixels 103 in which the datahas been written are brought into a holding state. Moreover, thetransistor 133 is connected to the capacitor 105; thus, the written datacan be held for a long time. Further, the amount of current flowingbetween the source electrode and the drain electrode of the transistor102 is controlled. The light-emitting element 131 emits light with aluminance corresponding to the amount of flowing current. This operationis sequentially performed row by row; thus, an image can be displayed.

A specific structure of an element substrate included in the displaydevice is described. Here, a specific example of a liquid crystaldisplay device including a liquid crystal element in the pixel 103 isdescribed. FIG. 2 is a top view of the pixel 103 illustrated in FIG. 1B.

In FIG. 2, a conductive film 13 serving as a scan line extendssubstantially perpendicularly to the signal line (in the horizontaldirection in the drawing). A conductive film 21 a serving as a signalline extends substantially perpendicularly to the scan line (in thevertical direction in the drawing). A conductive film 21 c serving as acapacitor line extends in parallel to the signal line. Note that theconductive film 13 serving as a scan line is electrically connected tothe scan line driver circuit 104 (see FIG. 1A), and the conductive film21 a serving as a signal line and the conductive film 21 c serving as acapacitor line are electrically connected to the signal line drivercircuit 106 (see FIG. 1A).

The transistor 102 is provided at a region where the scan line and thesignal line cross each other. The transistor 102 includes the conductivefilm 13 serving as a gate electrode; the gate insulating film (notillustrated in FIG. 2); an oxide semiconductor film 19 a where a channelregion is formed, over the gate insulating film; and the conductivefilms 21 a and 21 b serving as a pair of electrodes. The conductive film13 also serves as a scan line, and a region of the conductive film 13that overlaps with the oxide semiconductor film 19 a serves as the gateelectrode of the transistor 102. In addition, the conductive film 21 aalso serves as a signal line, and a region of the conductive film 21 athat overlaps the oxide semiconductor film 19 a serves as the sourceelectrode or the drain electrode of the transistor 102. Further, in thetop view of FIG. 2, an end portion of the scan line is located on theouter side than an end portion of the oxide semiconductor film 19 a.Thus, the scan line functions as a light-blocking film for blockinglight from a light source such as a backlight. For this reason, theoxide semiconductor film 19 a included in the transistor is notirradiated with light, so that a variation in the electricalcharacteristics of the transistor can be suppressed.

The conductive film 21 b is electrically connected to thelight-transmitting conductive film 29 that serves as a pixel electrode,through an opening 41.

The capacitor 105 is connected to the conductive film 21 c serving as acapacitor line. The capacitor 105 includes the film 19 b havingconductivity formed over the gate insulating film, a dielectric filmformed of a nitride insulating film formed over the transistor 102, anda light-transmitting conductive film 29 that serves as a pixelelectrode. The dielectric film is formed using an oxygen barrier film.The film 19 b having conductivity formed over the gate insulating filmhas a light-transmitting property. That is, the capacitor 105 has alight-transmitting property.

Thanks to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (covers a large area) in the pixel103. Thus, a semiconductor device having charge capacity increased whileimproving the aperture ratio, typically, to 50% or more, 55% or more, or60% or more can be obtained. For example, in a semiconductor device withhigh resolution such as a liquid crystal display device, the area of apixel is small and thus the area of a capacitor is also small. For thisreason, the charge capacity of the capacitor is small. However, sincethe capacitor 105 of this embodiment has a light-transmitting property,when it is provided in a pixel, enough charge capacity can be obtainedin the pixel and the aperture ratio can be improved. Typically, thecapacitor 105 can be favorably used in a high-resolution semiconductordevice with a pixel density of 200 ppi or more, 300 ppi or more, or 500ppi or more.

Further, according to one embodiment of the present invention, theaperture ratio can be improved even in a display device with highresolution, which makes it possible to use light from a light sourcesuch as a backlight efficiently, so that power consumption of thedisplay device can be reduced.

Next, FIG. 3 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 2. The transistor 102 shown in FIG. 2 is achannel-etched transistor. Note that the transistor 102 in the channellength direction, a connection portion between the transistor 102 andthe conductive film 29 serving as a pixel electrode, and the capacitor105 are illustrated in the cross-sectional view taken alongdashed-dotted line A-B, and the transistor 102 in the channel widthdirection is illustrated in the cross-sectional view taken alongdashed-dotted line C-D.

The transistor 102 in FIG. 3 has a single-gate structure and includesthe conductive film 13 serving as a gate electrode over the substrate11. In addition, the transistor 102 includes an oxygen barrier film 15formed over the substrate 11 and the conductive film 13 serving as agate electrode, an oxide insulating film 17 formed over the oxygenbarrier film 15, the oxide semiconductor film 19 a overlapping theconductive film 13 serving as a gate electrode with the oxygen barrierfilm 15 and the oxide insulating film 17 provided therebetween, and theconductive films 21 a and 21 b serving as a pair of electrodes which arein contact with the oxide semiconductor film 19 a. Moreover, an oxideinsulating film 23 is formed over the oxide insulating film 17, theoxide semiconductor film 19 a, and the conductive films 21 a and 21 bserving as a pair of electrodes, and an oxide insulating film 25 isformed over the oxide insulating film 23. An oxygen barrier film 27 isformed over the oxygen barrier film 15, the oxide insulating film 17,the oxide insulating film 23, the oxide insulating film 25, and theconductive films 21 a and 21 b. In addition, a conductive film 29connected to one of the conductive films 21 a and 21 b serving as a pairof electrodes, here the conductive film 21 b, is formed over the oxygenbarrier film 27. Note that the conductive film 29 serves as a pixelelectrode.

The capacitor 105 in FIG. 3 includes a film 19 b having conductivityformed over the oxide insulating film 17, the oxygen barrier film 27,and the conductive film 29 serving as a pixel electrode.

Over the transistor 102 in this embodiment, the oxide insulating films23 and 25, which are subjected to element isolation, are formed. Theoxide insulating films 23 and 25 overlap the oxide semiconductor film 19a. Furthermore, the oxygen barrier film 15 is in contact with the oxygenbarrier film 27 with the oxide semiconductor film 19 a and the oxideinsulating films 23 and 25 provided on an inner side of the oxygenbarrier films 15 and 27.

As the oxygen barrier films 15 and 27, insulating films each having alow oxygen-transmitting property can be used. Alternatively, aninsulating film that hardly permeates oxygen, hydrogen, and water can beused. As the insulating film having a low oxygen-transmitting propertyand the insulating film that hardly permeates oxygen, hydrogen, andwater, nitride insulating films such as a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, and an aluminumnitride oxide film are given. Alternatively, as the insulating filmhaving a low oxygen-transmitting property and the insulating film thathardly permeates oxygen, hydrogen, and water, oxide insulating filmssuch as an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, and a hafnium oxynitride film aregiven.

The oxide semiconductor film 19 a is formed using, typically, an In—Gaoxide film, an In—Zn oxide film, or an In-M-Zn oxide film (M representsAl, Ga, Y, Zr, La, Ce, or Nd).

The oxide insulating film 23 or the oxide insulating film 25 providedover the oxide semiconductor film 19 a is an oxide insulating film whichcontains more oxygen than that in the stoichiometric composition. Partof oxygen is released by heating from the oxide insulating filmcontaining more oxygen than that in the stoichiometric composition. Theoxide insulating film containing more oxygen than that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, or greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis in which heat treatment is performed such that a temperature ofa film surface is higher than or equal to 100° C. and lower than orequal to 700° C. or higher than or equal to 100° C. and lower than orequal to 500° C.

In the case where the oxide insulating film containing more oxygen thanthat in the stoichiometric composition is included in the oxideinsulating film 23 or the oxide insulating film 25, part of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 can be moved to the oxide semiconductor film 19 a, so that oxygenvacancies contained in the oxide semiconductor film 19 a can be reduced.

Furthermore, the oxygen barrier film 15 is in contact with the oxygenbarrier film 27 with the oxide semiconductor film 19 a and the oxideinsulating films 23 and 25 provided on an inner side of the oxygenbarrier films 15 and 27.

The threshold voltage of a transistor using an oxide semiconductor filmwhich contains oxygen vacancies easily shifts negatively, and such atransistor tends to be normally-on. This is because electric charges aregenerated owing to oxygen vacancies in the oxide semiconductor film andthe resistance is thus reduced. The transistor having normally-oncharacteristics causes various problems in that malfunction is likely tobe caused when in operation and that power consumption is increased whennot in operation, for example. Further, there is a problem in that theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor over time or due to a stress test isincreased.

However, in the transistor 102 in this embodiment, the oxide insulatingfilm 23 or the oxide insulating film 25 provided over the oxidesemiconductor film 19 a contains more oxygen than that in thestoichiometric composition. Moreover, the oxide semiconductor film 19 a,the oxide insulating film 23, and the oxide insulating film 25 aresurrounded by the oxygen barrier film 15 and the oxygen barrier film 27.As a result, oxygen contained in the oxide insulating film 23 or theoxide insulating film 25 is moved to the oxide semiconductor film 19 aefficiently, so that oxygen vacancies in the oxide semiconductor film 19a are reduced. Accordingly, a transistor having normally-offcharacteristics is obtained. Further, the amount of change in electricalcharacteristics, typically in threshold voltage, of the transistor overtime or due to a stress test can be reduced.

In the capacitor 105, the film 19 b having conductivity is formed at thesame time as the oxide semiconductor film 19 a and has increasedconductivity by containing impurities. Alternatively, the film 19 bhaving conductivity is formed at the same time as the oxidesemiconductor film 19 a, and has increased conductivity by containingimpurities and containing oxygen vacancies generated by plasma damage orthe like.

On an element substrate of the semiconductor device illustrated in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, theconductive film that serves as a pixel electrode is used as the otherelectrode of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the semiconductor device can be reduced. Further, sincethe pair of electrodes has a light-transmitting property, the capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Details of the transistor 102 are described below.

There is no particular limitation on a material and the like of thesubstrate 11 as long as the material has heat resistance high enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 11. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used. Still alternatively, any of thesesubstrates provided with a semiconductor element may be used as thesubstrate 11. In the case where a glass substrate is used as thesubstrate 11, a glass substrate having any of the following sizes can beused: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, alarge-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 11, andthe transistor 102 may be provided directly on the flexible substrate.Alternatively, a separation layer may be provided between the substrate11 and the transistor 102. The separation layer can be used when part orthe whole of a semiconductor device formed over the separation layer iscompleted and separated from the substrate 11 and transferred to anothersubstrate. In such a case, the transistor 102 can be transferred to asubstrate having low heat resistance or a flexible substrate as well.

The conductive film 13 serving as a gate electrode can be formed using ametal element selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten; an alloy containing any of thesemetal elements as a component; an alloy containing any of these metalelements in combination; or the like. Further, one or more metalelements selected from manganese or zirconium may be used. Theconductive film 13 serving as a gate electrode may have a single-layerstructure or a stacked structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which an aluminum film is stacked over a titaniumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a two-layer structure in which a copper film is stackedover a titanium film, a three-layer structure in which a titanium film,an aluminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, an alloy film or a nitride film whichcontains aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may beused.

The conductive film 13 serving as a gate electrode can also be formedusing a light-transmitting conductive material such as indium tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to have astacked-layer structure formed using the above light-transmittingconductive material and the above metal element.

As the oxygen barrier film 15, an insulating film having a lowoxygen-transmitting property can be used. Alternatively, an insulatingfilm that hardly permeates oxygen, hydrogen, and water can be used. Asthe insulating film having a low oxygen-transmitting property and theinsulating film that hardly permeates oxygen, hydrogen, and water,nitride insulating films such as a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, and an aluminum nitrideoxide film are given. Alternatively, as the insulating film having a lowoxygen-transmitting property and the insulating film that hardlypermeates oxygen, hydrogen, and water, oxide insulating films such as analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, and a hafnium oxynitride film are given.

The thickness of the oxygen barrier film 15 is preferably greater thanor equal to 5 nm and less than or equal to 100 nm, or greater than orequal to 20 nm and less than or equal to 80 nm.

The oxide insulating film 17 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, any ofsilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, and Ga—Zn-basedmetal oxide.

The oxide insulating film 17 may also be formed using a high-k materialsuch as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogenis added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen isadded (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so thatgate leakage current of the transistor can be reduced.

The thickness of the oxide insulating film 17 is greater than or equalto 5 nm and less than or equal to 400 nm, greater than or equal to 10 nmand less than or equal to 300 nm, or greater than or equal to 50 nm andless than or equal to 250 nm.

The oxide semiconductor film 19 a is typically formed using In—Ga oxide,In—Zn oxide, or an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, orNd).

Note that in the case where the oxide semiconductor film 19 a is anIn-M-Zn oxide film, the proportions of In and M when summation of In andM is assumed to be 100 atomic % are as follows: the atomic percentage ofIn is greater than or equal to 25 atomic % and the atomic percentage ofM is less than 75 atomic %; or the atomic percentage of In is greaterthan or equal to 34 atomic % and the atomic percentage of M is less than66 atomic %.

The energy gap of the oxide semiconductor film 19 a is 2 eV or more, 2.5eV or more, or 3 eV or more. With the use of an oxide semiconductorhaving such a wide energy gap, the off-state current of the transistor102 can be reduced.

The thickness of the oxide semiconductor film 19 a is greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to3 nm and less than or equal to 100 nm, or greater than or equal to 3 nmand less than or equal to 50 nm.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming the In-M-Zn oxide film satisfy In≧M and Zn≧M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that theproportion of each metal element in the atomic ratio of the oxidesemiconductor film 19 a to be formed varies within a range of ±40% ofthat in the above atomic ratio of the sputtering target as an error.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 19 a. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, 1×10¹⁵/cm³ or lower,1×10¹³/cm³ or lower, or 1×10¹¹/cm³ or lower is used as the oxidesemiconductor film 19 a.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. Further,in order to obtain required semiconductor characteristics of atransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 19 a be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 19 a,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “highly purified substantially intrinsic”. A highlypurified intrinsic or highly purified substantially intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor including the oxidesemiconductor film in which a channel region is formed rarely has anegative threshold voltage (is rarely normally-on). A highly purifiedintrinsic or highly purified substantially intrinsic oxide semiconductorfilm has a low density of defect states and accordingly has few carriertraps in some cases. Further, the highly purified intrinsic or highlypurified substantially intrinsic oxide semiconductor film has anextremely low off-state current; even when an element has a channelwidth of 1×10⁶ μm and a channel length (L) of 10 μM, the off-statecurrent can be less than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A,at a voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V. Thus, the transistor whose channel regionis formed in the oxide semiconductor film has a small variation inelectrical characteristics and high reliability in some cases. Chargestrapped by the trap states in the oxide semiconductor film take a longtime to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm having a high density of trap states has unstable electricalcharacteristics in some cases. Examples of the impurities includehydrogen, nitrogen, alkali metal, and alkaline earth metal.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductor film19 a. Specifically, the hydrogen concentration of the oxidesemiconductor film 19 a, which is measured by secondary ion massspectrometry (SIMS), is lower than or equal to 5×10¹⁹ atoms/cm³, lowerthan or equal to 1×10¹⁹ atoms/cm³, lower than or equal to 5×10¹⁸atoms/cm³, lower than or equal to 1×10¹⁸ atoms/cm³, lower than or equalto 5×10¹⁷ atoms/cm³, or lower than or equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 19 a, oxygen vacancies areincreased, and the oxide semiconductor film 19 a becomes an n-type film.Thus, the concentration of silicon or carbon (the concentration ismeasured by SIMS) of the oxide semiconductor film 19 a is lower than orequal to 2×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁷ atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 19 a, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁶atoms/cm³. Alkali metal and alkaline earth metal might generate carrierswhen bonded to an oxide semiconductor, in which case the off-statecurrent of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 19 a.

Further, when containing nitrogen, the oxide semiconductor film 19 aeasily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 a may have a non-single-crystalstructure, for example. The non-single crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure described later, or an amorphous structure, for example. Amongthe non-single crystal structure, the amorphous structure has thehighest density of defect levels, whereas CAAC-OS has the lowest densityof defect levels.

The oxide semiconductor film 19 a may have an amorphous structure, forexample. An oxide semiconductor film having an amorphous structure has,for example, disordered atomic arrangement and no crystalline component.Alternatively, an oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and has no crystal part.

Note that the oxide semiconductor film 19 a may be a mixed filmincluding two or more of the following: a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film includes, for example, two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases. Further, the mixed film has a stacked-layer structure of two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

The film 19 b having conductivity is formed by processing an oxidesemiconductor film formed at the same time as the oxide semiconductorfilm 19 a. Thus, the film 19 b having conductivity contains a metalelement similar to that in the oxide semiconductor film 19 a. Further,the film 19 b having conductivity has a crystal structure similar to ordifferent from that of the oxide semiconductor film 19 a. By addingimpurities or oxygen vacancies to the oxide semiconductor film formed atthe same time as the oxide semiconductor film 19 a, the film 19 b havingconductivity is formed. An example of the impurities contained in theoxide semiconductor film is hydrogen. Instead of hydrogen, as theimpurity, boron, phosphorus, tin, antimony, a rare gas element, analkali metal, an alkaline earth metal, or the like may be included.

The oxide semiconductor film 19 a and the film 19 b having conductivityare both formed over the oxide insulating film 17, but differ inimpurity concentration. Specifically, the film 19 b having conductivityhas a higher impurity concentration than the oxide semiconductor film 19a. For example, the concentration of hydrogen contained in the oxidesemiconductor film 19 a is lower than or equal to 5×10¹⁹ atoms/cm³,lower than or equal to 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸atoms/cm³, lower than or equal to 5×10¹⁷ atoms/cm³, or lower than orequal to 1×10¹⁶ atoms/cm³. The concentration of hydrogen contained inthe film 19 b having conductivity is higher than or equal to 8×10¹⁹atoms/cm³, higher than or equal to 1×10²⁰ atoms/cm³, or higher than orequal to 5×10²⁰ atoms/cm³. The concentration of hydrogen contained inthe film 19 b having conductivity is greater than or equal to 2 times orgreater than or equal to 10 times that in the oxide semiconductor film19 a.

On the other hand, when the oxide semiconductor film formed at the sametime as the oxide semiconductor film 19 a is exposed to plasma, theoxide semiconductor film is damaged, and oxygen vacancies can begenerated. For example, when a film is formed over the oxidesemiconductor film by a plasma CVD method or a sputtering method, theoxide semiconductor film is exposed to plasma and oxygen vacancies aregenerated. Alternatively, when the oxide semiconductor film is exposedto plasma in etching treatment for formation of the oxide insulatingfilm 23 and the oxide insulating film 25, oxygen vacancies aregenerated. Alternatively, when the oxide semiconductor film is exposedto plasma such as a mixed gas of oxygen and hydrogen, hydrogen, a raregas, and ammonia, oxygen vacancies are generated. As a result, theconductivity of the oxide semiconductor film is increased, so that thefilm 19 b having conductivity is formed.

That is, the film 19 b having conductivity can be called an oxidesemiconductor film having high conductivity. Alternatively, the film 19b having conductivity can be called a metal oxide film having highconductivity.

In the case where a silicon nitride film is used as the oxygen barrierfilm 27, the silicon nitride film contains hydrogen. Thus, when hydrogenin the oxygen barrier film 27 is diffused into the oxide semiconductorfilm formed at the same time as the oxide semiconductor film 19 a,hydrogen is bonded to oxygen and electrons serving as carriers aregenerated in the oxide semiconductor film. Further, when the siliconnitride film is formed as the oxygen barrier film 27 by a plasma CVDmethod or a sputtering method, the oxide semiconductor film is exposedto plasma, so that oxygen vacancies are generated. When hydrogencontained in the silicon nitride film enters the oxygen vacancies,electrons serving as carriers are generated. As a result, theconductivity of the oxide semiconductor film is increased, so that thefilm 19 b having conductivity is formed.

The film 19 b having conductivity has lower resistivity than the oxidesemiconductor film 19 a. The resistivity of the film 19 b havingconductivity is preferably greater than or equal to 1×10⁻⁸ times andless than 1×10⁻¹ times the resistivity of the oxide semiconductor film19 a. The resistivity of the film 19 b having conductivity is typicallygreater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, or greaterthan or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

The conductive films 21 a and 21 b serving as a pair of electrodes areeach formed to have a single-layer structure or a stacked-layerstructure including any of metals such as aluminum, titanium, chromium,nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, andtungsten or an alloy containing any of these metals as its maincomponent. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which an aluminumfilm is stacked over a tungsten film, a two-layer structure in which acopper film is stacked over a copper-magnesium-aluminum alloy film, atwo-layer structure in which a copper film is stacked over a titaniumfilm, a two-layer structure in which a copper film is stacked over atungsten film, a three-layer structure in which a titanium film or atitanium nitride film, an aluminum film or a copper film, and a titaniumfilm or a titanium nitride film are stacked in this order, a three-layerstructure in which a molybdenum film or a molybdenum nitride film, analuminum film or a copper film, and a molybdenum film or a molybdenumnitride film are stacked in this order, and the like can be given. Notethat a transparent conductive material containing indium oxide, tinoxide, or zinc oxide may be used.

As the oxide insulating film 23 or the oxide insulating film 25, anoxide insulating film which contains more oxygen than that in thestoichiometric composition is preferably used. Here, as the oxideinsulating film 23, an oxide insulating film which permeates oxygen isformed, and as the oxide insulating film 25, an oxide insulating filmwhich contains more oxygen than that in the stoichiometric compositionis formed.

The oxide insulating film 23 is an oxide insulating film which permeatesoxygen. Thus, oxygen released from the oxide insulating film 25 providedover the oxide insulating film 23 can be moved to the oxidesemiconductor film 19 a through the oxide insulating film 23. Moreover,the oxide insulating film 23 also serves as a film which relieves damageto the oxide semiconductor film 19 a at the time of forming the oxideinsulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, or greater than or equal to 5 nm and less than or equal to 50 nm canbe used as the oxide insulating film 23. Note that in thisspecification, a “silicon oxynitride film” refers to a film thatcontains oxygen at a higher proportion than nitrogen, and a “siliconnitride oxide film” refers to a film that contains nitrogen at a higherproportion than oxygen.

Further, it is preferable that the amount of defects in the oxideinsulating film 23 be small, typically the spin density of a signalwhich appears at g=2.001 due to a dangling bond of silicon, be lowerthan or equal to 3×10¹⁷ spins/cm³ by ESR measurement. This is because ifthe density of defects in the oxide insulating film 23 is high, oxygenis bonded to the defects and the amount of oxygen that passes throughthe oxide insulating film 23 is decreased.

Further, it is preferable that the amount of defects at the interfacebetween the oxide insulating film 23 and the oxide semiconductor film 19a be small, typically the spin density of a signal which appears atg=1.93 due to an oxygen vacancy in the oxide semiconductor film 19 a belower than or equal to 1×10¹⁷ spins/cm³, more preferably lower than orequal to the lower limit of detection by ESR measurement.

Note that in the oxide insulating film 23, all oxygen having entered theoxide insulating film 23 from the outside moves to the outside in somecases. Alternatively, some oxygen having entered the oxide insulatingfilm 23 from the outside remains in the oxide insulating film 23 in somecases. Further, movement of oxygen occurs in the oxide insulating film23 in some cases in such a manner that oxygen enters the oxideinsulating film 23 from the outside and oxygen contained in the oxideinsulating film 23 is moved to the outside of the oxide insulating film23.

The oxide insulating film 25 is formed in contact with the oxideinsulating film 23. The oxide insulating film 25 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm containing oxygen at a higher proportion than the stoichiometriccomposition is an oxide insulating film of which the amount of releasedoxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, or greaterthan or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis in which heattreatment is performed such that a temperature of a film surface ishigher than or equal to 100° C. and lower than or equal to 700° C. orhigher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, or greater than or equal to 50 nm and less than or equal to 400 nmcan be used as the oxide insulating film 25.

Further, it is preferable that the amount of defects in the oxideinsulating film 25 be small, typically the spin density of a signalwhich appears at g=2.001 originating from a dangling bond of silicon, belower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulating film25 is provided more apart from the oxide semiconductor film 19 a thanthe oxide insulating film 23 is; thus, the oxide insulating film 25 mayhave higher defect density than the oxide insulating film 23.

As the oxygen barrier film 27, an insulating film having a lowoxygen-transmitting property can be used. Alternatively, an insulatingfilm that hardly permeates oxygen, hydrogen, and water can be used. Asthe insulating film having a low oxygen-transmitting property and theinsulating film that hardly permeates oxygen, hydrogen, and water,nitride insulating films such as a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, and an aluminum nitrideoxide film are given. Alternatively, as the insulating film having a lowoxygen-transmitting property and the insulating film that hardlypermeates oxygen, hydrogen, and water, oxide insulating films such as analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, and a hafnium oxynitride film are given.

The oxygen barrier film 15 and the oxygen barrier film 27 are providedto be in contact with each other with the oxide semiconductor film 19 aand the oxide insulating films 23 and 25 provided on an inner side ofthe oxygen barrier films 15 and 27. Therefore, the movement of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 to the outside of the oxygen barrier film 15 and the oxygen barrierfilm 27 can be suppressed. As a result, oxygen contained in the oxideinsulating film 23 or the oxide insulating film 25 can be moved to theoxide semiconductor film 19 a efficiently, and the amount of oxygenvacancies in the oxide semiconductor film can be reduced.

The thickness of the oxygen barrier film 27 can be greater than or equalto 50 nm and less than or equal to 300 nm, or greater than or equal to100 nm and less than or equal to 200 nm.

The conductive film 29 is formed using a light-transmitting conductivefilm. As the light-transmitting conductive film, an indium oxide filmcontaining tungsten oxide, an indium zinc oxide film containing tungstenoxide, an indium oxide film containing titanium oxide, an indium tinoxide film containing titanium oxide, an indium tin oxide (hereinafter,referred to as ITO) film, an indium zinc oxide film, an indium tin oxidefilm to which silicon oxide is added, and the like are given.

Next, a method for manufacturing the transistor 102 and the capacitor105 in FIG. 3 is described with reference to FIGS. 4A to 4D, FIGS. 5A to5D, FIGS. 6A and 6B, and FIGS. 7A and 7B.

As illustrated in FIG. 4A, a conductive film 12 to be the conductivefilm 13 is formed over the substrate 11. The conductive film 12 isformed by a sputtering method, a CVD method, an evaporation method, orthe like.

Here, a glass substrate is used as the substrate 11. Further, as theconductive film 12, a 100 nm-thick tungsten film is formed by asputtering method.

Then, a mask is formed over the conductive film 12 by a photolithographyprocess using a first photomask. Next, as illustrated in FIG. 4B, partof the conductive film 12 is etched with the use of the mask to form theconductive film 13 serving as a gate electrode. After that, the mask isremoved.

Note that the conductive film 13 serving as a gate electrode may beformed by an electrolytic plating method, a printing method, an ink jetmethod, or the like instead of the above formation method.

Here, a tungsten film is etched by dry etching to form the conductivefilm 13 serving as a gate electrode.

Next, as illustrated in FIG. 4C, over the conductive film 13 serving asa gate electrode, the oxygen barrier film 15 and an oxide insulatingfilm 16 to be the oxide insulating film 17 later are formed. Then, overthe oxide insulating film 16, an oxide semiconductor film 18 to be theoxide semiconductor film 19 a and the film 19 b having conductivity isformed.

The oxygen barrier film 15 and the oxide insulating film 16 are formedby a sputtering method, a CVD method, an evaporation method, or thelike.

Here, as the oxygen barrier film 15, a 300-nm-thick silicon nitride filmis formed by a plasma CVD method in which silane, nitrogen, and ammoniaare used as a source gas.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the oxide insulating film 16, adeposition gas containing silicon and an oxidizing gas are preferred tobe used as a source gas. Typical examples of the deposition gascontaining silicon include silane, disilane, trisilane, and silanefluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide,nitrogen dioxide, and the like can be given as examples.

Moreover, in the case of forming a gallium oxide film as the oxideinsulating film 16, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Here, as the oxide insulating film 16, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane and dinitrogenmonoxide are used as a source gas.

The oxide semiconductor film 18 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, or the like.

In the case where the oxide semiconductor film is formed by a sputteringmethod, a power supply device for generating plasma can be an RF powersupply device, an AC power supply device, a DC power supply device, orthe like as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen to a rare gas is preferably increased.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film to be formed.

In order to obtain an intrinsic or substantially intrinsic oxidesemiconductor film, besides the high vacuum evacuation of the chamber, ahighly purification of a sputtering gas is also needed. As an oxygen gasor an argon gas used for a sputtering gas, a gas which is highlypurified to have a dew point of −40° C. or lower, −80° C. or lower,−100° C. or lower, or −120° C. or lower is used, whereby entry ofmoisture or the like into the oxide semiconductor film can be preventedas much as possible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=3:1:2).

Then, after a mask is formed over the oxide semiconductor film 18 by aphotolithography process using a second photomask, the oxidesemiconductor film is partly etched using the mask. Thus, the oxidesemiconductor films 19 a and 19 c subjected to element isolation asillustrated in FIG. 4D are formed. After that, the mask is removed.

Next, as illustrated in FIG. 5A, a conductive film 20 to be theconductive films 21 a, 21 b, and 21 c later is formed.

The conductive film 20 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film aresequentially stacked by a sputtering method.

Next, a mask is formed over the conductive film 20 by a photolithographyprocess using a third photomask. Then, the conductive film 20 is etchedwith the use of the mask, and as illustrated in FIG. 5B, the conductivefilms 21 a and 21 b serving as a pair of electrodes and the conductivefilm 21 c serving as a capacitor line are formed. After that, the maskis removed.

Here, a mask is formed over the copper film by a photolithographyprocess. Then, the tungsten film and the copper film are etched with theuse of the mask, so that the conductive films 21 a, 21 b, and 21 c areformed. Note that the copper film is etched by a wet etching method.Next, the tungsten film is etched by a dry etching method using SF₆,whereby fluoride is formed on the surface of the copper film. By thefluoride, diffusion of copper elements from the copper film is reducedand thus the copper concentration in the oxide semiconductor film 19 acan be reduced.

Next, as illustrated in FIG. 5C, an oxide insulating film 22 to be theoxide insulating film 23 later and an oxide insulating film 24 to be theoxide insulating film 25 later are formed over the oxide semiconductorfilms 19 a and 19 c and the conductive films 21 a, 21 b, and 21 c.

Note that after the oxide insulating film 22 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 22 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby theconcentration of impurities attributed to the atmospheric component atthe interface between the oxide insulating film 22 and the oxideinsulating film 24 can be reduced and oxygen in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 19 a; accordingly,the amount of oxygen vacancies in the oxide semiconductor film 19 a canbe reduced.

As the oxide insulating film 22, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure is greater thanor equal to 20 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 250 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power issupplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 22. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

With the use of the above conditions, an oxide insulating film whichpermeates oxygen can be formed as the oxide insulating film 22. Further,by providing the oxide insulating film 22, damage to the oxidesemiconductor film 19 a can be reduced in a step of forming the oxideinsulating film 25 which is formed later.

A silicon oxide film or a silicon oxynitride film can be formed as theoxide insulating film 22 under the following conditions: the substrateplaced in a treatment chamber of a plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 280°C. and lower than or equal to 400° C., the pressure is greater than orequal to 100 Pa and less than or equal to 250 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power issupplied to an electrode provided in the treatment chamber.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the oxide insulating film 22, a dense and hard oxideinsulating film which permeates oxygen, typically, a silicon oxide filmor a silicon oxynitride film of which etching using hydrofluoric acid of0.5 wt % at 25° C. is performed at a rate lower than or equal to 10nm/min, or lower than or equal to 8 nm/min can be formed.

The oxide insulating film 22 is formed while heating is performed; thus,hydrogen, water, or the like contained in the oxide semiconductor film19 a can be released in the step. Hydrogen contained in the oxidesemiconductor film 19 a is bonded to an oxygen radical formed in plasmato form water. Since the substrate is heated in the step of forming theoxide insulating film 22, water formed by bonding of oxygen and hydrogenis released from the oxide semiconductor film. That is, when the oxideinsulating film 22 is formed by a plasma CVD method, the amount of waterand hydrogen contained in the oxide semiconductor film 19 a can bereduced.

Further, time for heating in a state where the oxide semiconductor film19 a is exposed can be shortened because heating is performed in a stepof forming the oxide insulating film 22. Thus, the amount of oxygenreleased from the oxide semiconductor film by heat treatment can bereduced. That is, the amount of oxygen vacancies in the oxidesemiconductor film can be reduced.

Note that by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, the amount ofwater contained in the oxide insulating film 22 is reduced; thus,variation in electrical characteristics of the transistor 102 can bereduced and change in threshold voltage can be inhibited.

Further, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, damage to theoxide semiconductor film 19 a can be reduced when the oxide insulatingfilm 22 is formed, so that the amount of oxygen vacancies contained inthe oxide semiconductor film 19 a can be reduced. In particular, whenthe film formation temperature of the oxide insulating film 22 or theoxide insulating film 24 which is formed later is set to be high,typically higher than 220° C., part of oxygen contained in the oxidesemiconductor film 19 a is released and oxygen vacancies are easilyformed. Further, when the film formation conditions for reducing theamount of defects in the oxide insulating film 24 which is formed laterare used to increase reliability of the transistor, the amount ofreleased oxygen is easily reduced. Thus, it is difficult to reduceoxygen vacancies in the oxide semiconductor film 19 a in some cases.However, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa to reducedamage to the oxide semiconductor film 19 a at the time of forming theoxide insulating film 22, oxygen vacancies in the oxide semiconductorfilm 19 a can be reduced even when the amount of oxygen released fromthe oxide insulating film 24 is small.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the oxide insulating film 22 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilm 19 a can be reduced; thus, the negative shift in the thresholdvoltage of the transistor can be inhibited.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 150W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film which permeates oxygen can be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., or higher than or equal to 200°C. and lower than or equal to 240° C., the pressure is greater than orequal to 100 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power ofgreater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm²,or greater than or equal to 0.25 W/cm² and less than or equal to 0.35W/cm² is supplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the oxide insulating film 24 becomes higher than thatin the stoichiometric composition. On the other hand, in the film formedat a substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in the later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Further, the oxide insulatingfilm 22 is provided over the oxide semiconductor film 19 a. Accordingly,in the step of forming the oxide insulating film 24, the oxideinsulating film 22 serves as a protective film of the oxidesemiconductor film 19 a. Consequently, the oxide insulating film 24 canbe formed using the high-frequency power having a high power densitywhile damage to the oxide semiconductor film 19 a is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and the high-frequency power of1500 W is supplied to the parallel-plate electrodes with the use of a27.12 MHz high-frequency power source. Note that a plasma CVD apparatusused here is a parallel-plate plasma CVD apparatus in which theelectrode area is 6000 cm², and the power per unit area (power density)into which the supplied power is converted is 0.25 W/cm².

Further, when the conductive films 21 a and 21 b serving as a pair ofelectrodes is formed, the oxide semiconductor film 19 a is damaged bythe etching of the conductive film, so that oxygen vacancies aregenerated on the back channel side (the side of the oxide semiconductorfilm 19 a which is opposite to the side facing to the conductive film 13serving as a gate electrode) of the oxide semiconductor film 19 a.However, with the use of the oxide insulating film containing oxygen ata higher proportion than the stoichiometric composition as the oxideinsulating film 24, the oxygen vacancies generated on the back channelside can be repaired by heat treatment. By this, defects contained inthe oxide semiconductor film 19 a can be reduced, and thus, thereliability of the transistor 102 can be improved.

Then, a mask is formed over the oxide insulating film 24 by aphotolithography process using a fourth photomask. Next, as illustratedin FIG. 5D, part of the oxide insulating film 22 and part of the oxideinsulating film 24 are etched with the use of the mask to form the oxideinsulating film 23 and the oxide insulating film 25. After that, themask is removed.

In the process, the oxide insulating films 22 and 24 are preferablyetched by a dry etching method. As a result, the oxide semiconductorfilm 19 c is exposed to plasma in the etching treatment; thus, oxygenvacancies in the oxide semiconductor film 19 c can be increased.

The oxide insulating films 22 and 24 are etched so that end portions ofthe oxide insulating films 23 and 25 are positioned on an outer side ofthe oxide semiconductor film 19 a in the channel length direction asillustrated in the cross-sectional view taken along the dashed-dottedline A-B, and end portions of the oxide insulating films 23 and 25 arepositioned on an outer side of the oxide semiconductor film 19 a in thechannel width direction as illustrated in the cross-sectional view takenalong the dashed-dotted line C-D. As a result, the oxide insulatingfilms 23 and 25, which are subjected to element isolation, can beformed. Part of the oxide insulating film 16 is etched at the same timeas the etching of the oxide insulating film 23, so that the oxideinsulating film 17 can be formed. As a result, the oxygen barrier film15 is exposed.

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., higher than or equal to 300° C. and lower thanor equal to 400° C., or higher than or equal to 320° C. and lower thanor equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less, 1ppm or less, or 10 ppb or less), or a rare gas (argon, helium, or thelike). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 25 can be moved to the oxide semiconductor film 19 a, so that theoxygen vacancies contained in the oxide semiconductor film 19 a can bereduced.

In the case where water, hydrogen, or the like is contained in the oxideinsulating film 23 and the oxide insulating film 25 and a film 26 havinga barrier property against oxygen which is formed later also has abarrier property against water, hydrogen, or the like, when the film 26having a barrier property is formed later and heat treatment isperformed, water, hydrogen, or the like contained in the oxideinsulating film 23 and the oxide insulating film 25 are moved to theoxide semiconductor film 19 a, so that defects are generated in theoxide semiconductor film 19 a. However, by the heating, water, hydrogen,or the like contained in the oxide insulating film 23 and the oxideinsulating film 25 can be released; thus, variation in electricalcharacteristics of the transistor 102 can be reduced, and change inthreshold voltage can be inhibited.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 22 while being heated, oxygen can be moved to the oxidesemiconductor film 19 a to reduce the oxygen vacancies in the oxidesemiconductor film 19 a; thus, the heat treatment is not necessarilyperformed.

The heat treatment may be performed after the formation of the oxideinsulating films 22 and 24. However, the heat treatment is preferablyperformed after the formation of the oxide insulating films 23 and 25because a film having conductivity can be formed in such a manner thatoxygen is not moved to the oxide semiconductor film 19 c and oxygen isreleased from the oxide semiconductor film 19 c because of exposure ofthe oxide semiconductor film 19 c and then oxygen vacancies aregenerated.

Here, heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

Next, as illustrated in FIG. 6A, the film 26 to be the oxygen barrierfilm 27 later is formed.

The film 26 to be the oxygen barrier film 27 later is formed by asputtering method, a CVD method, or the like. By forming the film 26 tobe the oxygen barrier film 27 later by a sputtering method, a CVDmethod, or the like, the oxide semiconductor film 19 c is exposed toplasma; thus, oxygen vacancies in the oxide semiconductor film 19 c canbe increased.

Through the process, the oxygen barrier film 15 is in contact with thefilm 26 to be the oxygen barrier film 27 with the oxide semiconductorfilm 19 a, the oxide insulating film 23, and the oxide insulating film25 provided on an inner side of the oxygen barrier film 15 and the film26 to be the oxygen barrier film 27.

In addition, the oxide semiconductor film 19 c becomes the film 19 bhaving conductivity. In the case where a silicon nitride film is formedby a plasma CVD method as the film 26 to be the oxygen barrier film 27,hydrogen in the silicon nitride film is diffused to the oxidesemiconductor film 19 c, so that the film 19 b having high conductivitycan be formed.

Note that in the case where a silicon nitride film is formed by a plasmaCVD method as the film 26 to be the oxygen barrier film 27, thesubstrate placed in the treatment chamber of the plasma CVD apparatusthat is vacuum-evacuated is preferably kept at a temperature higher thanor equal to 300° C. and lower than or equal to 400° C., or higher thanor equal to 320° C. and lower than or equal to 370° C., so that a densesilicon nitride film can be formed.

In the case where a silicon nitride film is formed, a deposition gascontaining silicon, nitrogen, and ammonia are preferably used as asource gas. As the source gas, a small amount of ammonia compared to theamount of nitrogen is used, whereby ammonia is dissociated in the plasmaand activated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which a bondbetween silicon and nitrogen is promoted and a bond between silicon andhydrogen is few can be formed. On the other hand, when the amount ofammonia with respect to nitrogen is large in a source gas, cleavage of adeposition gas containing silicon and cleavage of nitrogen are notpromoted, so that a sparse silicon nitride film in which a bond betweensilicon and hydrogen remains and defects are increased is formed.Therefore, in a source gas, a flow ratio of the nitrogen to the ammoniais set to be greater than or equal to 5 and less than or equal to 50, orgreater than or equal to 10 and less than or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed as the film 26 to be the oxygen barrierfilm 27 by a plasma CVD method in which silane with a flow rate of 50sccm, nitrogen with a flow rate of 5000 sccm, and ammonia with a flowrate of 100 sccm are used as the source gas, the pressure in thetreatment chamber is 100 Pa, the substrate temperature is 350° C., andhigh-frequency power of 1000 W is supplied to parallel-plate electrodeswith a high-frequency power supply of 27.12 MHz. Note that the plasmaCVD apparatus is a parallel-plate plasma CVD apparatus in which theelectrode area is 6000 cm², and the power per unit area (power density)into which the supplied power is converted is 1.7×10⁻¹ W/cm².

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature higher than or equal to 150° C. and lowerthan or equal to 400° C., higher than or equal to 300° C. and lower thanor equal to 400° C., or higher than or equal to 320° C. and lower thanor equal to 370° C. Since the oxide semiconductor film 19 a and theoxide insulating films 23 and 25 are provided in a region surrounded bythe oxygen barrier films 15 and 26 which are in contact with each other,the movement of oxygen from the oxide semiconductor film 19 a and theoxide insulating films 23 and 25 to the outside can be prevented. As aresult, the negative shift of the threshold voltage can be reduced.Moreover, the amount of change in the threshold voltage can be reduced.

Then, after a mask is formed over the film 26 to be the oxygen barrierfilm 27 later by a photolithography process using a fifth photomask, thefilm 26 to be the oxygen barrier film 27 later is etched using the mask.Thus, the oxygen barrier film 27 having an opening 41 as illustrated inFIG. 6B is formed.

Next, as illustrated in FIG. 7A, a conductive film 28 to be theconductive film 29 later is formed over the conductive film 21 b and theoxygen barrier film 27.

The conductive film 28 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 7B, partof the conductive film 28 is etched with the use of the mask to form theconductive film 29. After that, the mask is removed.

Through the above process, the transistor 102 is manufactured and thecapacitor 105 can also be manufactured.

In the transistor in this embodiment, the oxygen barrier film 15 is incontact with the oxygen barrier film 27 with the oxide semiconductorfilm 19 a and the oxide insulating films 23 and 25 provided on an innerside of the oxygen barrier films 15 and 27. At least one of the oxideinsulating film 23 and the oxide insulating film 25 is formed using anoxide insulating film which contains more oxygen than that in thestoichiometric composition. Therefore, the movement of oxygen containedin the oxide insulating film 23 or the oxide insulating film 25 to theoutside of the oxygen barrier film 15 and the oxygen barrier film 27 canbe suppressed. As a result, oxygen contained in the oxide insulatingfilm 23 or the oxide insulating film 25 can be moved to the oxidesemiconductor film 19 a efficiently, and the amount of oxygen vacanciesin the oxide semiconductor film 19 a can be reduced.

On an element substrate of the semiconductor device illustrated in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, theconductive film that serves as a pixel electrode is used as the otherelectrode of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the semiconductor device can be reduced. Further, sincethe pair of electrodes has a light-transmitting property, the capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

In this embodiment, the oxide insulating films to be the oxideinsulating films 23 and 25 are formed by a plasma CVD method in whichheating is performed at a temperature of higher than or equal to 280° C.and lower than or equal to 400° C. Thus, hydrogen, water, or the likecontained in the oxide semiconductor film 19 a can be released. Further,in the step, the length of heating time in a state where the oxidesemiconductor film is exposed is short, and even when the temperature ofheat treatment is lower than or equal to 400° C., it is possible tomanufacture a transistor in which the amount of change in thresholdvoltage is equivalent to that of a transistor subjected to heattreatment at a high temperature. Consequently, the manufacturing cost ofa semiconductor device can be reduced.

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device with improved electricalcharacteristics can be obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a semiconductor device and a manufacturing methodthereof, which are different from those shown in Embodiment 1, aredescribed with reference to the drawings. The semiconductor device inthis embodiment is different from that in Embodiment 1 in that thetransistor has a structure in which an oxide semiconductor film isprovided between different gate electrodes, that is, a dual-gatestructure. Note that the description about the same structures as thosein Embodiment 1 is omitted.

A specific structure of an element substrate included in the displaydevice is described. Here, a specific example of a liquid crystaldisplay device including a liquid crystal element in the pixel 103 isdescribed. FIG. 8 is a top view of the pixel 103 illustrated in FIG. 1B.

The structure in this embodiment is different from that in Embodiment 1in that, in a plan view of the pixel 103 in FIG. 8, a conductive film 29a serving as a gate electrode and overlapping part of or the whole ofeach of the conductive film 13 serving as a gate electrode, the oxidesemiconductor film 19 a, the conductive films 21 a and 21 b, and theoxide insulating film 25 is provided. The conductive film 29 a servingas a gate electrode is connected to the conductive film 13 serving as agate electrode in an opening 41 a.

Next, FIG. 9 is a cross-sectional view taken along dashed-dotted linesA-B and C-D in FIG. 8. A transistor 102 a shown in FIG. 9 is achannel-etched transistor. Note that the transistor 102 a in the channellength direction, a connection portion between the transistor 102 a andthe conductive film 29 serving as a pixel electrode, and the capacitor105 a are illustrated in the cross-sectional view taken alongdashed-dotted line A-B, and the transistor 102 a in the channel widthdirection and a cross-sectional view of a connection portion between theconductive film 13 and the conductive film 29 a each serving as a gateelectrode are illustrated in the cross-sectional view taken alongdashed-dotted line C-D.

The transistor 102 a in FIG. 9 has a dual-gate structure and includesthe conductive film 13 serving as a gate electrode over the substrate11. In addition, the transistor 102 a includes the oxygen barrier film15 formed over the substrate 11 and the conductive film 13 serving as agate electrode, an oxide insulating film 17 formed over the oxygenbarrier film 15, the oxide semiconductor film 19 a overlapping theconductive film 13 serving as a gate electrode with the oxygen barrierfilm 15 and the oxide insulating film 17 provided therebetween, and theconductive films 21 a and 21 b which serve as a pair of electrodes andare in contact with the oxide semiconductor film 19 a. Moreover, theoxide insulating film 23 is formed over the oxide insulating film 17,the oxide semiconductor film 19 a, and the conductive films 21 a and 21b serving as a pair of electrodes, and the oxide insulating film 25 isformed over the oxide insulating film 23. The oxygen barrier film 27 isformed over the oxygen barrier film 15, the oxide insulating film 17,the oxide insulating film 23, the oxide insulating film 25, and theconductive films 21 a and 21 b. In addition, the conductive film 29connected to one of the conductive films 21 a and 21 b serving as a pairof electrodes, here the conductive film 21 b, and the conductive film 29a serving as a gate electrode are formed over the oxygen barrier film27.

As illustrated in the cross-sectional view taken along the dashed-dottedline C-D, the conductive film 29 a serving as a gate electrode isconnected to the conductive film 13 serving as a gate electrode in theopening 41 a provided in the oxygen barrier films 15 and 27. That is,the conductive film 13 serving as a gate electrode and the conductivefilm 29 a serving as a gate electrode have the same potential.

Thus, by applying voltage at the same potential to each gate electrodeof the transistor 102 a, variation in the initial characteristics can bereduced, and degradation of the transistor 102 a after the −GBT stresstest and a change in the rising voltage of on-state current at differentdrain voltages can be suppressed. In addition, a region where carriersflow in the oxide semiconductor film 19 a becomes larger in the filmthickness direction, so that the amount of carrier movement isincreased. As a result, the on-state current of the transistor 102 a isincreased and the field-effect mobility becomes higher, typicallybecomes higher than or equal to 20 cm²/V·s.

Over the transistor 102 a in this embodiment, the oxide insulating films23 and 25, which are subjected to element isolation, are formed. Theoxide insulating films 23 and 25, which are subjected to elementisolation, overlap the oxide semiconductor film 19 a. In thecross-sectional view in the channel width direction, end portions of theoxide insulating films 23 and 25 are positioned on an outer side thanthe oxide semiconductor film 19 a. Furthermore, in the channel widthdirection in FIG. 9, the conductive film 29 a serving as a gateelectrode faces a side surface of the oxide semiconductor film 19 a withthe oxide insulating films 23 and 25 provided therebetween.

An end portion processed by etching or the like of the oxidesemiconductor film, in which defects are generated by damage due toprocessing, is also contaminated by the attachment of an impurity, orthe like. Thus, the end portion of the oxide semiconductor film iseasily activated by application of a stress such as an electric field,thereby easily becoming n-type (having a low resistance). Therefore, theend portion of the oxide semiconductor film 19 a overlapping theconductive film 13 serving as a gate electrode easily becomes n-type.When the end portion which becomes n-type is provided between theconductive films 21 a and 21 b serving as the pair of electrodes, theregion which becomes n-type serves as a carrier path, resulting in aparasitic channel. However, as illustrated in the cross-sectional viewtaken along the dashed-dotted line C-D, when the conductive film 29 aserving as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 a serving as a gate electrode,generation of a parasitic channel on the side surface of the oxidesemiconductor film 19 a or in a region including the side surface andthe vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

The oxide insulating film 23 or the oxide insulating film 25 providedover the oxide semiconductor film 19 a is an oxide insulating film whichcontains more oxygen than that in the stoichiometric composition.

In the case where the oxide insulating film containing more oxygen thanthat in the stoichiometric composition is included in the oxideinsulating film 23 or the oxide insulating film 25, part of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 can be moved to the oxide semiconductor film 19 a, so that oxygenvacancies contained in the oxide semiconductor film 19 a can be reduced.

The oxygen barrier film 15 and the oxygen barrier film 27 are providedto be in contact with each other with the oxide semiconductor film 19 aand the oxide insulating films 23 and 25 provided on an inner side ofthe oxygen barrier films 15 and 27. Therefore, the movement of oxygencontained in the oxide insulating film 23 or the oxide insulating film25 to the outside of the oxygen barrier film 15 and the oxygen barrierfilm 27 can be suppressed. As a result, oxygen contained in the oxideinsulating film 23 or the oxide insulating film 25 can be moved to theoxide semiconductor film 19 a efficiently, and the amount of oxygenvacancies in the oxide semiconductor film can be reduced.

The threshold voltage of a transistor using an oxide semiconductor filmwhich contains oxygen vacancies easily shifts negatively, and such atransistor tends to be normally-on. This is because electric charges aregenerated owing to oxygen vacancies in the oxide semiconductor film andthe resistance is thus reduced. The transistor having normally-oncharacteristics causes various problems in that malfunction is likely tobe caused when in operation and that power consumption is increased whennot in operation, for example. Further, there is a problem in that theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor over time or due to a stress test isincreased.

However, in the transistor 102 a in this embodiment, the oxideinsulating film 23 or the oxide insulating film 25 provided over theoxide semiconductor film 19 a contains more oxygen than that in thestoichiometric composition. Moreover, the oxide semiconductor film 19 a,the oxide insulating film 23, and the oxide insulating film 25 aresurrounded by the oxygen barrier film 15 and the oxygen barrier film 27.As a result, oxygen contained in the oxide insulating film 23 or theoxide insulating film 25 is moved to the oxide semiconductor film 19 aefficiently, so that oxygen vacancies in the oxide semiconductor film 19a can be reduced. Accordingly, a transistor having normally-offcharacteristics is obtained. Further, the amount of change in electricalcharacteristics, typically in threshold voltage, of the transistor overtime or due to a stress test can be reduced.

In the capacitor 105 a, the film 19 b having conductivity is formed atthe same time as the oxide semiconductor film 19 a and has highconductivity owing to impurities. Alternatively, the film 19 b havingconductivity is formed at the same time as the oxide semiconductor film19 a, and has high conductivity by containing impurities and oxygenvacancies generated by plasma damage or the like.

On an element substrate of the semiconductor device illustrated in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, theconductive film that serves as a pixel electrode is used as the otherelectrode of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the semiconductor device can be reduced. Further, sincethe pair of electrodes has a light-transmitting property, the capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

Details of the transistor 102 a are described below. Note that thedescription of the components with the same reference numerals as thosein Embodiment 1 is omitted.

The conductive film 29 a serving as a gate electrode can be formed usinga material similar to that of the conductive film 29 in Embodiment 1.

Next, a method for manufacturing the transistor 102 a and the capacitor105 a in FIG. 9 is described with reference to FIGS. 4A to 4D, FIGS. 5Ato 5D, FIG. 6A, and FIGS. 10A to 10C.

In a manner similar to that of Embodiment 1, through the processes ofFIGS. 4A to 4D, FIGS. 5A to 5D, and FIG. 6A, the conductive film 13serving as a gate electrode, the oxygen barrier film 15, the oxideinsulating film 16, the oxide semiconductor film 19 a, the film 19 bhaving conductivity, the conductive films 21 a and 21 b serving as apair of electrodes, the oxide insulating film 23, the oxide insulatingfilm 25, and the film 26 to be the oxygen barrier film 27 are formedover the substrate 11. In these processes, photography processes usingthe first photomask to the fourth photomask are performed.

Then, after a mask is formed over the film 26 to be the oxygen barrierfilm 27 by a photolithography process using a fifth photomask, part ofthe film 26 to be the oxygen barrier film 27 is etched using the mask.Thus, the oxygen barrier film 27 having the opening 41 and an opening 41a as illustrated in FIG. 10A is formed.

Next, as illustrated in FIG. 10B, the conductive film 28 to be theconductive films 29 and 29 a later is formed over the conductive film 13serving as a gate electrode, the conductive film 21 b, and the oxygenbarrier film 27.

Then, a mask is formed over the conductive film 28 by a photolithographyprocess using a sixth photomask. Next, as illustrated in FIG. 10C, partof the conductive film 28 is etched with the use of the mask to form theconductive film 29 serving as a pixel electrode and the conductive film29 a serving as a gate electrode. After that, the mask is removed.

Through the above process, the transistor 102 a is manufactured and thecapacitor 105 a can also be manufactured.

In the transistor described in this embodiment, when the conductive film29 a serving as a gate electrode faces a side surface of the oxidesemiconductor film 19 a with the oxide insulating films 23 and 25provided therebetween in the channel width direction, due to theelectric field of the conductive film 29 a serving as a gate electrode,generation of a parasitic channel on the side surface of the oxidesemiconductor film 19 a or in a region including the side surface andthe vicinity of the side surface is suppressed. As a result, atransistor which has excellent electrical characteristics such as asharp increase in the drain current at the threshold voltage isobtained.

In the transistor in this embodiment, the oxygen barrier film 15 is incontact with the oxygen barrier film 27 with the oxide semiconductorfilm 19 a and the oxide insulating films 23 and 25 provided on an innerside of the oxygen barrier films 15 and 27. At least one of the oxideinsulating film 23 and the oxide insulating film 25 is formed using anoxide insulating film which contains more oxygen than that in thestoichiometric composition. Therefore, the movement of oxygen containedin the oxide insulating film 23 or the oxide insulating film 25 to theoutside of the oxygen barrier film 15 and the oxygen barrier film 27 canbe suppressed. As a result, oxygen contained in the oxide insulatingfilm 23 or the oxide insulating film 25 can be moved to the oxidesemiconductor film 19 a efficiently, and the amount of oxygen vacanciesin the oxide semiconductor film 19 a can be reduced.

On an element substrate of the semiconductor device illustrated in thisembodiment, one electrode of the capacitor is formed at the same time asthe oxide semiconductor film of the transistor. In addition, theconductive film that serves as a pixel electrode is used as the otherelectrode of the capacitor. Thus, a step of forming another conductivefilm is not needed to form the capacitor, and the number of steps ofmanufacturing the semiconductor device can be reduced. Further, sincethe pair of electrodes has a light-transmitting property, the capacitorhas a light-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

In this embodiment, the oxide insulating films to be the oxideinsulating films 23 and 25 are formed by a plasma CVD method in whichheating is performed at a temperature of higher than or equal to 280° C.and lower than or equal to 400° C. Thus, hydrogen, water, or the likecontained in the oxide semiconductor film 19 a can be released. Further,in the step, the length of heating time in a state where the oxidesemiconductor film is exposed is short, and even when the temperature ofheat treatment is lower than or equal to 400° C., it is possible tomanufacture a transistor in which the amount of change in thresholdvoltage is equivalent to that of a transistor subjected to heattreatment at a high temperature. Consequently, the manufacturing cost ofa semiconductor device can be reduced.

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device with improved electricalcharacteristics can be obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, the electrical characteristics of the transistorhaving a dual-gate structure described in Embodiment 2 that includesgate electrodes connected to each other and having the same potentialare described with reference to FIG. 9 and FIGS. 11A to 16C.

Note that here a driving method in which the conductive films 13 and 29a serving as gate electrodes in FIG. 9 are electrically short-circuitedand are supplied with a gate voltage is referred to as dual-gatedriving. In other words, in dual-gate driving, the voltages of theconductive films 13 and 29 a serving as gate electrodes are always equalto each other.

Here, the electrical characteristics of the transistor were evaluated.FIGS. 11A and 11B illustrate the structures of transistors used for thecalculation. Note that device simulation software “Atlas” produced bySilvaco Inc. was used for the calculation.

A transistor having Structure 1 in FIG. 11A is a dual-gate transistor.

In the transistor having Structure 1, an insulating film 203 is formedover a gate electrode 201, and an oxide semiconductor film 205 is formedover the insulating film 203. A pair of electrodes 207 and 208 areformed over the insulating film 203 and the oxide semiconductor film205, and an insulating film 209 is formed over the oxide semiconductorfilm 205 and the pair of electrodes 207 and 208. A gate electrode 213 isformed over the insulating film 209. The gate electrode 201 is connectedto the gate electrode 213 at an opening (not illustrated) formed in theinsulating films 203 and 209.

A transistor having Structure 2 in FIG. 11B is a single-gate transistor.

In the transistor having Structure 2, the insulating film 203 is formedover the gate electrode 201, and the oxide semiconductor film 205 isformed over the insulating film 203. The pair of electrodes 207 and 208are formed over the insulating film 203 and the oxide semiconductor film205, and the insulating film 209 is formed over the oxide semiconductorfilm 205 and the pair of electrodes 207 and 208.

Note that in the calculation, the work function φ_(M) of the gateelectrode 201 was set to 5.0 eV. The insulating film 203 was a100-nm-thick film having a dielectric constant of 4.1. The oxidesemiconductor film 205 was a single-layer In—Ga—Zn oxide film(In:Ga:Zn=1:1:1). The band gap E_(g) of the In—Ga—Zn oxide film was 3.15eV, the electron affinity χ was 4.6 eV, the dielectric constant was 15,the electron mobility was 10 cm²/Vs, and the donor density N_(d) was3×10¹⁷ atoms/cm³. The work function φ_(sd) of the pair of electrodes 207and 208 was set to 4.6 eV. Ohmic junction was made between the oxidesemiconductor film 205 and each of the pair of electrodes 207 and 208.The insulating film 209 was a 100-nm-thick film having a dielectricconstant of 4.1. Note that defect levels, surface scattering, and thelike in the oxide semiconductor film 205 were not considered. Further,the channel length and the channel width of the transistor were 10 μmand 100 μm, respectively.

<Reduction in Variation in Initial Characteristics>

As in the case of the transistor having Structure 1, by the dual-gatedriving, variation in initial characteristics can be reduced. This isbecause on account of the dual-gate driving, the amount of change in thethreshold voltage V_(th), which is one of Id-Vg characteristics, of thetransistor having Structure 1 can be small as compared to that of thetransistor having Structure 2.

Here, as one example, a negative shift in the threshold voltage of theId-Vg characteristics that is caused because a semiconductor filmbecomes n-type is described.

The sum of the amount of charges of donor ions in the oxidesemiconductor film is Q (C), the capacitance formed by the gateelectrode 201, the insulating film 203, and the oxide semiconductor film205 is C_(Bottom), the capacitance formed by the oxide semiconductorfilm 205, the insulating film 209, and the gate electrode 213 isC_(Top). The amount of change ΔV of V_(th) of the transistor havingStructure 1 in that case is expressed by Formula 1. The amount of changeΔV of V_(th) of the transistor having Structure 2 in that case isexpressed by Formula 2.

$\begin{matrix}{{\Delta\; V} = {- \frac{Q}{C_{Bottom} + C_{Top}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack \\{{\Delta\; V} = {- \frac{Q}{C_{Bottom}}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

As expressed by Formula 1, by the dual-gate driving performed in thetransistor having Structure 1, the capacitance between donor ions in theoxide semiconductor film and the gate electrode becomes the sum ofC_(Bottom) and C_(Top); thus, the amount of change in the thresholdvoltage is small.

FIG. 12A shows the calculation results of the current-voltage curves atdrain voltages of 0.1 V and 1 V of the transistor having Structure 1.FIG. 12B shows the calculation results of the current-voltage curves atdrain voltages of 0.1 V and 1 V of the transistor having Structure 2.When the drain voltage Vd is 0.1 V, the threshold voltage of thetransistor having Structure 1 is −2.26 V and the threshold voltage ofthe transistor having Structure 2 is −4.73 V.

As in the case of the transistor having Structure 1, when the dual-gatedriving is employed, the amount of change of the threshold voltage canbe small. Thus, variation in electrical characteristics among aplurality of transistors can also be small.

Note that although a negative shift in the threshold voltage due to thedonor ions in the oxide semiconductor film is considered here, apositive shift in the threshold voltage due to fixed charges, mobilecharges, or negative charges (electrons trapped by acceptor-like states)in the insulating films 203 and 209 is similarly suppressed, which mightreduce the variation.

<Reduction in Degradation Due to −GBT Stress Test>

By the dual-gate driving performed in the transistor having Structure 1,degree of degradation due to a −GBT stress test can be low. Some reasonswhy degree of degradation due to a −GBT stress test can be low aredescribed below.

One of the reasons is that electrostatic stress is not caused on accountof the dual-gate driving. FIG. 13A is a diagram in which potentialcontour lines are plotted in the case where −30 V is applied to each ofthe gate electrodes 201 and 213 in the transistor having Structure 1.FIG. 13B shows potentials at the cross section A-B in FIG. 13A.

The oxide semiconductor film 205 is an intrinsic semiconductor, and whena negative voltage is applied to the gate electrodes 201 and 213 and theoxide semiconductor film 205 is fully depleted, no charge exists betweenthe gate electrodes 201 and 213. With this state, when the samepotential is supplied to the gate electrodes 201 and 213, as illustratedin FIG. 13B, the potential of the gate electrode 201 becomes completelyequal to that of the gate electrode 213. Since the potentials are equalto each other, electrostatic stress is not caused on the insulating film203, the oxide semiconductor film 205, and the insulating film 209. As aresult, phenomena causing degradation due to the −GBT stress test, suchas mobile ions and trap and detrap of carriers in the insulating films203 and 209, do not occur.

Another reason is that an external electric field of an FET can beblocked in the case of the dual-gate driving. FIG. 14A illustrates amodel in which charged particles in the air are adsorbed on the gateelectrode 213 in the transistor having Structure 1 illustrated in FIG.11A. FIG. 14B illustrates a model in which charged particles in the airare adsorbed on the insulating film 209 in the transistor havingStructure 2 illustrated in FIG. 11B.

As illustrated in FIG. 14B, in the transistor having Structure 2,positively charged particles in the air are adsorbed on a surface of theinsulating film 209. When a negative voltage is applied to the gateelectrode 201, positively charged particles are adsorbed on theinsulating film 209. As a result, as indicated by arrows in FIG. 14B, anelectric field of the positively charged particles affects the interfaceof the oxide semiconductor film 205 with the insulating film 209, sothat a state similar to the state when a positive bias is applied isbrought about. As a result, the threshold voltage might shift in thenegative direction.

In contrast, even if positively charged particles are adsorbed on asurface of the gate electrode 213 in the transistor having Structure 1illustrated in FIG. 14A, as indicated by arrows in FIG. 14A, the gateelectrode 213 blocks the electric field of the positively chargedparticles; thus, the positively charged particles do not affect theelectrical characteristics of the transistor. In sum, the transistor canbe electrically protected against external charges by the gate electrode213, leading to suppression of the degradation due to the −GBT stresstest.

For the above two reasons, in the transistor operated by the dual-gatedriving, the degradation due to the −GBT stress test can be suppressed.

<Suppression of Changes in Rising Voltages of on-State Current atDifferent Drain Voltages>

Here, in the case of Structure 2, changes in the rising voltages ofon-state current at different drain voltages and a cause of the changesare described.

In a transistor illustrated in FIGS. 15A to 15C, a gate insulating film233 is provided over a gate electrode 231, and an oxide semiconductorfilm 235 is provided over the gate insulating film 233. A pair ofelectrodes 237 and 238 are provided over the oxide semiconductor film235, and an insulating film 239 is provided over the gate insulatingfilm 233, the oxide semiconductor film 235, and the pair of electrodes237 and 238.

Note that in the calculation, the work function φ_(M) of the gateelectrode 231 was set to 5.0 eV. The gate insulating film 233 had astacked-layer structure including a 400-nm-thick film having adielectric constant of 7.5 and a 50-nm-thick film having a dielectricconstant of 4.1. The oxide semiconductor film 235 was a single-layerIn—Ga—Zn oxide film (In:Ga:Zn=1:1:1). The band gap E_(g) of the In—Ga—Znoxide film was 3.15 eV, the electron affinity χ was 4.6 eV, thedielectric constant was 15, the electron mobility was 10 cm²/Vs, and thedonor density N_(d) was 1×10¹³/cm³. The work function φ_(sd) of the pairof electrodes 237 and 238 was set to 4.6 eV. Ohmic junction was madebetween the oxide semiconductor film 235 and each of the pair ofelectrodes 207 and 208. The insulating film 239 was a 550-nm-thick filmhaving a dielectric constant of 3.9. Note that defect levels, surfacescattering, and the like in the oxide semiconductor film 235 were notconsidered. Further, the channel length and the channel width of thetransistor were 3 μm and 50 μm, respectively.

Next, models of a transistor illustrated in FIG. 15A in which positivelycharged particles are adsorbed on a surface of the insulating film 239are illustrated in FIGS. 15B and 15C. FIG. 15B illustrates an assumedstructure in which positive fixed charges are uniformly adsorbed on thesurface of the insulating film 239. FIG. 15C illustrates an assumedstructure in which positive fixed charges are partly adsorbed on thesurface of the insulating film 239.

Calculation results of the electrical characteristics of the transistorsillustrated in FIGS. 15A to 15C are shown in FIGS. 16A to 16C,respectively.

In the case where it is assumed that no positive fixed charge isadsorbed on the insulating film 239 in the transistor illustrated inFIG. 15A, the rising voltage at a drain voltage V_(d) of 1 Vapproximately corresponds to that at a drain voltage V_(d) of 10 V asshown in FIG. 16A.

In contrast, in the case where it is assumed that positive fixed chargesare uniformly adsorbed on the insulating film 239 in the transistorillustrated in FIG. 15B, the threshold voltages shift in the negativedirection and the rising voltage at a drain voltage V_(d) of 1 Vapproximately corresponds to that at a drain voltage V_(d) of 10 V asshown in FIG. 16B.

In the case where it is assumed that positive fixed charges are partlyadsorbed on the insulating film 239 in the transistor illustrated inFIG. 15C, the rising voltage at a drain voltage V_(d) of 1 V isdifferent from that at a drain voltage V_(d) of 10 V as shown in FIG.16C.

Since the gate electrode 213 is provided in the transistor havingStructure 1, as described in <Reduction in degradation due to −GBTstress test>, the gate electrode 213 blocks the electric field ofexternal charged particles; thus, the charged particles do not affectthe electrical characteristics of the transistor. In other words, thetransistor can be electrically protected against external charges by thegate electrode 213, and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed.

As described above, in the case where a dual-gate structure is employedand a given voltage is given to each gate electrode, degradation due toa −GBT stress test and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed. Moreover, in thecase where a dual-gate structure is employed and voltages having thesame potential are given to each gate electrode, variation in initialcharacteristics can be reduced, degradation due to a −GBT stress testcan be suppressed, and changes in the rising voltages of on-statecurrent at different drain voltages can be suppressed.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 4

In each of the transistors described in Embodiments 1 to 3, a baseinsulating film can be provided between the substrate 11 and theconductive film 13 serving as a gate electrode as necessary. As amaterial of the base insulating film, silicon oxide, silicon oxynitride,silicon nitride, silicon nitride oxide, gallium oxide, hafnium oxide,yttrium oxide, aluminum oxide, aluminum oxynitride, and the like can begiven as examples. Note that when silicon nitride, gallium oxide,hafnium oxide, yttrium oxide, aluminum oxide, or the like is used as amaterial of the base insulating film, it is possible to suppressdiffusion of impurities such as alkali metal, water, and hydrogen intothe oxide semiconductor film 19 a from the substrate 11.

The base insulating film can be formed by a sputtering method, a CVDmethod, or the like.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 5

As for the conductive films 21 a and 21 b serving as a pair ofelectrodes provided in each of the transistors described in Embodiments1 to 4, it is possible to use a conductive material which is easilybonded to oxygen, such as tungsten, titanium, aluminum, copper,molybdenum, chromium, or tantalum, or an alloy thereof. Thus, oxygencontained in the oxide semiconductor film 19 a and the conductivematerial contained in the conductive films 21 a and 21 b serving as apair of electrodes are bonded to each other, so that an oxygen deficientregion is formed in the oxide semiconductor film 19 a. Further, in somecases, part of constituent elements of the conductive material thatforms the conductive films 21 a and 21 b serving as a pair of electrodesis mixed into the oxide semiconductor film 19 a. Consequently, as shownin FIG. 17, low-resistance regions 19 d and 19 e are formed in thevicinity of regions of the oxide semiconductor film 19 a which are incontact with the conductive films 21 a and 21 b serving as a pair ofelectrodes. The low-resistance regions 19 d and 19 e are formed betweenthe oxide insulating film 17 and the conductive films 21 a and 21 bserving as a pair of electrodes so as to be in contact with theconductive films 21 a and 21 b serving as a pair of electrodes. Sincethe low-resistance regions 19 d and 19 e have high conductivity, contactresistance between the oxide semiconductor film 19 a and the conductivefilms 21 a and 21 b serving as a pair of electrodes can be reduced, andthus, the on-state current of the transistor can be increased.

Further, the conductive films 21 a and 21 b serving as a pair ofelectrodes may each have a stacked-layer structure of the conductivematerial which is easily bonded to oxygen and a conductive materialwhich is not easily bonded to oxygen, such as titanium nitride, tantalumnitride, or ruthenium. With such a stacked-layer structure, oxidizationof the conductive films 21 a and 21 b serving as a pair of electrodescan be prevented at the interface between the conductive films 21 a and21 b serving as a pair of electrodes and the oxide insulating film 23,so that the increase of the resistance of the conductive films 21 a and21 b serving as a pair of electrodes can be inhibited.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 6

In this embodiment, a semiconductor device including a transistor inwhich the amount of defects in an oxide semiconductor film can befurther reduced as compared to Embodiments 1 and 2 is described withreference to drawings. Transistors described in this embodiment aredifferent from those in Embodiments 1 and 2 in that a multilayer film inwhich oxide semiconductor films are stacked is included. Here, detailsare described using the transistor in Embodiment 1.

FIGS. 18A and 18B each illustrate a cross-sectional view of an elementsubstrate included in a semiconductor device. FIGS. 18A and 18B arecross-sectional views taken along the dashed-dotted lines A-B and C-D inFIG. 2.

A transistor 102 b in FIG. 18A includes a multilayer film 37 aoverlapping the conductive film 13 serving as a gate electrode with theoxygen barrier film 15 and the oxide insulating film 17 providedtherebetween and the conductive films 21 a and 21 b serving as a pair ofelectrodes in contact with the multilayer film 37 a. Moreover, over theoxygen barrier film 15, the oxide insulating film 17, the multilayerfilm 37 a, and the conductive films 21 a and 21 b serving as a pair ofelectrodes, the oxide insulating film 23, the oxide insulating film 25,and the oxygen barrier film 27 are formed.

A capacitor 105 b in FIG. 18A includes a multilayer film 37 b formedover the oxide insulating film 17, the oxygen barrier film 27 in contactwith the multilayer film 37 b, and the conductive film 29 in contactwith the oxygen barrier film 27. The multilayer film 37 b is in contactwith the conductive film 21 c serving as a capacitor line. The oxygenbarrier film 15 and the oxygen barrier film 27 are in contact with eachother, and the multilayer film 37 b is provided between the oxygenbarrier film 15 and the oxygen barrier film 27.

In the transistor 102 b described in this embodiment, the multilayerfilm 37 a includes the oxide semiconductor film 19 a and an oxidesemiconductor film 39 a. That is, the multilayer film 37 a has atwo-layer structure. Further, part of the oxide semiconductor film 19 aserves as a channel region. Furthermore, the oxide insulating film 23 isformed in contact with the oxide semiconductor film 39 a, and the oxideinsulating film 25 is formed in contact with the oxide insulating film23. That is, the oxide semiconductor film 39 a is provided between theoxide semiconductor film 19 a and the oxide insulating film 23.

The oxide semiconductor film 39 a is a film containing one or moreelements that form the oxide semiconductor film 19 a. Thus, interfacescattering is unlikely to occur at the interface between the oxidesemiconductor films 19 a and 39 a. Thus, the transistor can have a highfield-effect mobility because the movement of carriers is not hinderedat the interface.

The oxide semiconductor film 39 a is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr,La, Ce, or Nd). The energy at the conduction band bottom of the oxidesemiconductor film 39 a is closer to a vacuum level than that of theoxide semiconductor film 19 a is, and typically, the difference betweenthe energy at the conduction band bottom of the oxide semiconductor film39 a and the energy at the conduction band bottom of the oxidesemiconductor film 19 a is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV orless, 0.5 eV or less, or 0.4 eV or less. That is, the difference betweenthe electron affinity of the oxide semiconductor film 39 a and theelectron affinity of the oxide semiconductor film 19 a is any one of0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more,and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV orless.

The oxide semiconductor film 39 a preferably contains In because carriermobility (electron mobility) can be increased.

When the oxide semiconductor film 39 a contains a larger amount of Al,Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in anatomic ratio, any of the following effects may be obtained: (1) theenergy gap of the oxide semiconductor film 39 a is widened; (2) theelectron affinity of the oxide semiconductor film 39 a decreases; (3) animpurity from the outside is blocked; (4) an insulating property of theoxide semiconductor film 39 a increases as compared to that of the oxidesemiconductor film 19 a; and (5) oxygen vacancies are less likely to begenerated because Al, Ga, Y, Zr, La, Ce, or Nd is a metal element thatis strongly bonded to oxygen.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are as follows: the atomic percentage of In is lessthan 50 atomic % and the atomic percentage of M is greater than or equalto 50 atomic %; or the atomic percentage of In is less than 25 atomic %and the atomic percentage of M is greater than or equal to 75 atomic %.

Further, in the case where each of the oxide semiconductor films 19 aand 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce,or Nd), the proportion of M atoms (M represents Al, Ga, Y, Zr, La, Ce,or Nd) in the oxide semiconductor film 39 a is higher than that in theoxide semiconductor film 19 a. Typically, the proportion of Min in theoxide semiconductor film 39 a is 1.5 or more times, twice or more, orthree or more times as high as that in the oxide semiconductor film 19a.

Furthermore, in the case where each of the oxide semiconductor films 19a and 39 a is an In-M-Zn-based oxide film (M represents Al, Ga, Y, Zr,La, Ce, or Nd), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] is satisfied in theoxide semiconductor film 39 a and In:M:Zn=x₂:y₂:z₂ [atomic ratio] issatisfied in the oxide semiconductor film 19 a, y₁/x₁ is higher thany₂/x₂, or y₁/x₁ be 1.5 or more times as high as y₂/x₂. Alternatively,y₁/x₁ be twice or more as high as y₂/x₂. Further alternatively, y₁/x₁ bethree or more times as high as y₂/x₂. In this case, it is preferablethat in the oxide semiconductor film, y₂ be higher than or equal to x₂because a transistor including the oxide semiconductor film can havestable electrical characteristics.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for forming theoxide semiconductor film 19 a, x₁/y₁ is preferably greater than or equalto ⅓ and less than or equal to 6, further preferably greater than orequal to 1 and less than or equal to 6, and z₁/y₁ is preferably greaterthan or equal to ⅓ and less than or equal to 6, further preferablygreater than or equal to 1 and less than or equal to 6. Note that whenz₁/y₁ is greater than or equal to 1 and less than or equal to 6, aCAAC-OS film to be described later as the oxide semiconductor film 19 ais easily formed. Typical examples of the atomic ratio of the metalelements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, andIn:M:Zn=3:1:2.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxidefilm (M is Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomicratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for forming theoxide semiconductor film 39 a, x₂/y₂ is preferably less than x₁/y₁, andz₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₂/y₂ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film to be described later as the oxidesemiconductor film 39 a is easily formed. Typical examples of the atomicratio of the metal elements of the target are In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, and the like.

Note that the proportion of each metal element in the atomic ratio ofeach of the oxide semiconductor films 19 a and the oxide semiconductorfilm 39 a varies within a range of ±40% of that in the above atomicratio as an error.

The oxide semiconductor film 39 a also serves as a film that relievesdamage to the oxide semiconductor film 19 a at the time of forming theoxide insulating film 25 later.

The thickness of the oxide semiconductor film 39 a is greater than orequal to 3 nm and less than or equal to 100 nm, or greater than or equalto 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 39 a may have a non-single-crystalstructure, for example, like the oxide semiconductor film 19 a. Thenon-single crystal structure includes a CAAC-OS that is described later,a polycrystalline structure, a microcrystalline structure describedlater, or an amorphous structure, for example.

The oxide semiconductor film 39 a may have an amorphous structure, forexample. An amorphous oxide semiconductor film, for example, hasdisordered atomic arrangement and no crystalline component.Alternatively, an amorphous oxide film is, for example, absolutelyamorphous and has no crystal part.

Note that the oxide semiconductor films 19 a and 39 a may each be amixed film including two or more of the following: a region having anamorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film has asingle-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, in some cases, the mixed film has a stacked-layer structure inwhich two or more of the following regions are stacked: a region havingan amorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure.

Here, the oxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23. Hence, if trapstates are formed between the oxide semiconductor film 39 a and theoxide insulating film 23 owing to impurities and defects, electronsflowing in the oxide semiconductor film 19 a are less likely to becaptured by the trap states because there is a distance between the trapstates and the oxide semiconductor film 19 a. Accordingly, the amount ofon-state current of the transistor can be increased, and thefield-effect mobility can be increased. When the electrons are capturedby the trap states, the electrons become negative fixed charges. As aresult, a threshold voltage of the transistor varies. However, by thedistance between the oxide semiconductor film 19 a and the trap states,capture of the electrons by the trap states can be reduced, andaccordingly change in the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 39 a. Consequently, the impurity concentrationand the number of oxygen vacancies in the oxide semiconductor film 19 acan be reduced.

Note that the oxide semiconductor films 19 a and 39 a are not formed bysimply stacking each film, but are formed to form a continuous junction(here, in particular, a structure in which the energy of the bottom ofthe conduction band is changed continuously between each film). In otherwords, a stacked-layer structure in which there exist no impurity thatforms a defect level such as a trap center or a recombination center ateach interface is provided. If an impurity exists between the oxidesemiconductor films 19 a and 39 a that are stacked, a continuity of theenergy band is damaged, and the carrier is captured or recombined at theinterface and then disappears.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to the air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 102 c in FIG. 18B, a multilayer film 38 a may beprovided instead of the multilayer film 37 a.

In addition, as in a capacitor 105 c in FIG. 18B, a multilayer film 38 bmay be provided instead of the multilayer film 37 b.

The multilayer film 38 a includes an oxide semiconductor film 49 a, theoxide semiconductor film 19 a, and the oxide semiconductor film 39 a.That is, the multilayer film 38 a has a three-layer structure. Further,the oxide semiconductor film 19 a serves as a channel region.

Further, the oxide insulating film 17 and the oxide semiconductor film49 a are in contact with each other. That is, the oxide semiconductorfilm 49 a is provided between the oxide insulating film 17 and the oxidesemiconductor film 19 a.

The multilayer film 38 a and the oxide insulating film 23 are in contactwith each other. In addition, the oxide semiconductor film 39 a and theoxide insulating film 23 are in contact with each other. That is, theoxide semiconductor film 39 a is provided between the oxidesemiconductor film 19 a and the oxide insulating film 23.

The oxide film 49 a can be formed using a material and a formationmethod similar to those of the oxide semiconductor film 39 a.

It is preferable that the thickness of the oxide film 49 a be smallerthan that of the oxide semiconductor film 19 a. When the thickness ofthe oxide film 49 a is greater than or equal to 1 nm and less than orequal to 5 nm, preferably greater than or equal to 1 nm and less than orequal to 3 nm, the amount of change in threshold voltage of thetransistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductorfilm 39 a is provided between the oxide semiconductor film 19 a and theoxide insulating film 23. Hence, if trap states are formed between theoxide semiconductor film 39 a and the oxide insulating film 23 owing toimpurities and defects, electrons flowing in the oxide semiconductorfilm 19 a are less likely to be captured by the trap states becausethere is a distance between the trap states and the oxide semiconductorfilm 19 a. Accordingly, the amount of on-state current of the transistorcan be increased, and the field-effect mobility can be increased. Whenthe electrons are captured by the trap states, the electrons becomenegative fixed charges. As a result, a threshold voltage of thetransistor varies. However, by the distance between the oxidesemiconductor film 19 a and the trap states, capture of the electrons bythe trap states can be reduced, and accordingly change in the thresholdvoltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductorfilm 39 a, and accordingly, the amount of impurities that aretransferred from the outside to the oxide semiconductor film 19 a can bereduced. Further, an oxygen vacancy is less likely to be formed in theoxide semiconductor film 39 a. Consequently, the impurity concentrationand the number of oxygen vacancies in the oxide semiconductor film 19 acan be reduced.

Further, the oxide film 49 a is provided between the oxide insulatingfilm 17 and the oxide semiconductor film 19 a, and the oxidesemiconductor film 39 a is provided between the oxide semiconductor film19 a and the oxide insulating film 23. Thus, it is possible to reducethe concentration of silicon or carbon in the vicinity of the interfacebetween the oxide semiconductor film 49 a and the oxide semiconductorfilm 19 a, the concentration of silicon or carbon in the oxidesemiconductor film 19 a, or the concentration of silicon or carbon inthe vicinity of the interface between the oxide semiconductor film 39 aand the oxide semiconductor film 19 a. Consequently, in the multilayerfilm 38 a, the absorption coefficient derived from a constantphotocurrent method is lower than 1×10⁻³/cm or lower than 1×10⁻⁴/cm, andthus density of localized levels is extremely low.

The transistors 102 b and 102 c each having such a structure includevery few defects in the multilayer film 38 a including the oxidesemiconductor film 32, thus, the electrical characteristics of thesetransistors can be improved, and typically, the on-state current can beincreased and the field-effect mobility can be improved. Further, in aBT stress test and a BT photostress test that are examples of a stresstest, the amount of change in threshold voltage is small, and thus,reliability is high.

<Band Structure of Transistor>

Next, band structures of the multilayer film 37 a provided in thetransistor 102 b illustrated in FIG. 18A and the multilayer film 38 aprovided in the transistor 102 c illustrated in FIG. 18B are describedwith reference to FIGS. 19A to 19C.

Here, for example, In—Ga—Zn oxide having an energy gap of 3.15 eV isused as the oxide semiconductor film 19 a, and In—Ga—Zn oxide having anenergy gap of 3.5 eV is used as the oxide semiconductor film 39 a. Theenergy gaps can be measured using a spectroscopic ellipsometer (UT-300manufactured by HORIBA JOBIN YVON SAS.).

The energy difference between the vacuum level and the top of thevalence band (also called ionization potential) of the oxidesemiconductor film 19 a and the energy difference between the vacuumlevel and the top of the valence band of the oxide semiconductor film 39a are 8 eV and 8.2 eV, respectively. Note that the energy differencebetween the vacuum level and the valence band top can be measured usingan ultraviolet photoelectron spectroscopy (UPS) device (VersaProbemanufactured by ULVAC-PHI, Inc.).

Thus, the energy difference between the vacuum level and the bottom ofthe conduction band (also called electron affinity) of the oxidesemiconductor film 19 a and the energy gap therebetween of the oxidesemiconductor film 39 a are 4.85 eV and 4.7 eV, respectively.

FIG. 19A schematically illustrates a part of the band structure of themultilayer film 37 a. Here, the case where a silicon oxide film isprovided in contact with the multilayer film 37 a is described. In FIG.19A, EcI1 denotes the energy of the bottom of the conduction band in thesilicon oxide film; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 19 a; EcS2 denotes theenergy of the bottom of the conduction band in the oxide semiconductorfilm 39 a; and EcI2 denotes the energy of the bottom of the conductionband in the silicon oxide film. Further, EcI1 and EcI2 correspond to theoxide insulating film 17 and the oxide insulating film 23 in FIG. 18B,respectively.

As illustrated in FIG. 19A, there is no energy barrier between the oxidesemiconductor films 19 a and 39 a, and the energy level of the bottom ofthe conduction band gradually changes therebetween. In other words, theenergy level of the bottom of the conduction band is continuouslychanged. This is because the multilayer film 37 a contains an elementcontained in the oxide semiconductor film 19 a and oxygen is transferredbetween the oxide semiconductor films 19 a and 39 a, so that a mixedlayer is formed.

As shown in FIG. 19A, the oxide semiconductor film 19 a in themultilayer film 37 a serves as a well and a channel region of thetransistor including the multilayer film 37 a is formed in the oxidesemiconductor film 19 a. Note that since the energy of the bottom of theconduction band of the multilayer film 37 a is continuously changed, itcan be said that the oxide semiconductor films 19 a and 39 a arecontinuous.

Although trap states due to impurities or defects might be formed in thevicinity of the interface between the oxide semiconductor film 39 a andthe oxide insulating film 23 as shown in FIG. 19A, the oxidesemiconductor film 19 a can be distanced from the trap states owing tothe existence of the oxide semiconductor film 39 a. However, when theenergy difference between EcS1 and EcS2 is small, an electron in theoxide semiconductor film 19 a might reach the trap state across theenergy difference. When the electron is captured by the trap state, anegative fixed charge is generated at the interface with the insulatingfilm, whereby the threshold voltage of the transistor shifts in thepositive direction. Therefore, it is preferable that the energydifference between EcS1 and EcS2 be 0.1 eV or more, more preferably 0.15eV or more, because change in the threshold voltage of the transistor isreduced and stable electrical characteristics are obtained.

FIG. 19B schematically illustrates a part of the band structure of themultilayer film 37 a, which is a variation of the band structure shownin FIG. 19A. Here, a structure where silicon oxide films are provided incontact with the multilayer film 37 a is described. In FIG. 19B, EcI1denotes the energy of the bottom of the conduction band in the siliconoxide film; EcS1 denotes the energy of the bottom of the conduction bandin the oxide semiconductor film 19 a; and EcI2 denotes the energy of thebottom of the conduction band in the silicon oxide film. Further, EcI1and EcI2 correspond to the oxide insulating film 17 and the oxideinsulating film 23 in FIG. 18B, respectively.

In the transistor illustrated in FIG. 18B, an upper portion of themultilayer film 37 a, that is, the oxide semiconductor film 39 a mightbe etched in formation of the conductive films 21 a and 21 b serving asa pair of electrodes. Further, a mixed layer of the oxide semiconductorfilms 19 a and 39 a is likely to be formed on the top surface of theoxide semiconductor film 19 a in formation of the oxide semiconductorfilm 39 a.

For example, when the oxide semiconductor film 19 a is an oxidesemiconductor film formed with use of, as a sputtering target, In—Ga—Znoxide whose atomic ratio of In to Ga and Zn is 1:1:1 or In—Ga—Zn oxidewhose atomic ratio of In to Ga and Zn is 3:1:2, and the oxidesemiconductor film 39 a is an oxide film formed with use of, as asputtering target, In—Ga—Zn oxide whose atomic ratio of In to Ga and Znis 1:3:2, In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4,or In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, the Gacontent in oxide semiconductor film 39 a is higher than that in theoxide semiconductor film 19 a. Thus, a GaOx layer or a mixed layer whoseGa content is higher than that in the oxide semiconductor film 19 a canbe formed on the top surface of the oxide semiconductor film 19 a.

For that reason, even in the case where oxide semiconductor film 39 a isetched, the energy of the bottom of the conduction band of EcS1 on theEcI2 side is increased and the band structure shown in FIG. 19B can beobtained in some cases.

As in the band structure shown in FIG. 19B, in observation of a crosssection of a channel region, only the oxide semiconductor film 19 a inthe multilayer film 37 a is apparently observed in some cases. However,a mixed layer that contains Ga more than the oxide semiconductor film 19a does is formed over the oxide semiconductor film 19 a in fact, andthus the mixed layer can be regarded as a 1.5-th layer. Note that themixed layer can be confirmed by analyzing a composition in the upperportion of the oxide semiconductor film 19 a, when the elementscontained in the multilayer film 37 a are measured by an EDX analysis,for example. The mixed layer can be confirmed, for example, in such amanner that the Ga content in the composition in the upper portion ofthe oxide semiconductor film 19 a is larger than the Ga content in theoxide semiconductor film 19 a.

FIG. 19C schematically illustrates a part of the band structure of themultilayer film 38 a. Here, the case where silicon oxide films areprovided in contact with the multilayer film 38 a is described. In FIG.19C, EcI1 denotes the energy of the bottom of the conduction band in thesilicon oxide film; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 19 a; EcS2 denotes theenergy of the bottom of the conduction oxide semiconductor film 39 a;EcS3 denotes the energy of the bottom of the conduction oxidesemiconductor film 49 a; and EcI2 denotes the energy of the bottom ofthe conduction band in the silicon oxide film. Further, EcI1 and EcI2correspond to the oxide insulating film 17 and the oxide insulating film23 in FIG. 18B, respectively.

As illustrated in FIG. 19C, there is no energy barrier between the oxidesemiconductor films 49 a, 19 a, and 39 a, and the energy level of thebottom of the conduction band gradually changes therebetween. In otherwords, the energy level of the bottom of the conduction band iscontinuously changed. This is because the multilayer film 38 a containsan element contained in the oxide semiconductor film 19 a and oxygen istransferred between the oxide semiconductor films 19 a and 49 a andbetween the oxide semiconductor films 19 a and 39 a, so that a mixedlayer is formed.

As shown in FIG. 19C, the oxide semiconductor film 19 a in themultilayer film 38 a serves as a well and a channel region of thetransistor including the multilayer film 38 a is formed in the oxidesemiconductor film 19 a. Note that since the energy of the bottom of theconduction band of the multilayer film 38 a is continuously changed, itcan be said that the oxide semiconductor films 49 a, 19 a, and 39 a arecontinuous.

Although, in the case where the oxide insulating film 17, the oxidesemiconductor film 19 a, and the oxide insulating film 23 are stacked inthis order, trap states due to impurities or defects might be formed inthe vicinity of the interface between the oxide semiconductor film 19 aand the oxide insulating film 23 and in the vicinity of the interfacebetween the oxide semiconductor film 19 a and the oxide insulating film17, as illustrated in FIG. 19C, the oxide semiconductor film 19 a can bedistanced from the trap states owing to the existence of the oxidesemiconductor films 39 a and 49 a. However, when the energy differencebetween EcS1 and EcS2 and the energy difference between EcS1 and EcS3are small, electrons in the oxide semiconductor film 19 a might reachthe trap state across the energy difference. When the electrons arecaptured by the trap state, a negative fixed charge is generated at theinterface with the oxide insulating film, whereby the threshold voltageof the transistor shifts in the positive direction. Therefore, it ispreferable that the energy difference between EcS1 and EcS2 and theenergy difference between EcS1 and EcS3 be 0.1 eV or more, morepreferably 0.15 eV or more, because change in the threshold voltage ofthe transistor is reduced and stable electrical characteristics areobtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 7

In this embodiment, one embodiment that can be applied to an oxidesemiconductor film in the transistor included in the semiconductordevice described in the above embodiment is described.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor), and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may be formed of aCAAC-OS film. Furthermore, the oxide semiconductor film may include anamorphous oxide semiconductor and an oxide semiconductor having acrystal grain. Described below are the single-crystal oxidesemiconductor, the CAAC-OS, the polycrystalline oxide semiconductor, themicrocrystalline oxide semiconductor, and the amorphous oxidesemiconductor.

<Single Crystal Oxide Semiconductor>

The single-crystal oxide semiconductor film has a lower impurityconcentration and a lower density of defect states (few oxygenvacancies). Thus, the carrier density can be decreased. Accordingly, atransistor including the single-crystal oxide semiconductor film isunlikely to be normally on. Moreover, since the single-crystal oxidesemiconductor film has a lower impurity concentration and a lowerdensity of defect states, carrier traps might be reduced. Thus, thetransistor including the single-crystal oxide semiconductor film hassmall variation in electrical characteristics and accordingly has highreliability.

Note that when the oxide semiconductor film has few defects, the densitythereof is increased. When the oxide semiconductor film has highcrystallinity, the density thereof is increased. When the oxidesemiconductor film has a lower concentration of impurities such ashydrogen, the density thereof is increased. The single-crystal oxidesemiconductor film has a higher density than the CAAC-OS film. TheCAAC-OS film has a higher density than the microcrystalline oxidesemiconductor film. The polycrystalline oxide semiconductor film has ahigher density than the microcrystalline oxide semiconductor film. Themicrocrystalline oxide semiconductor film has a higher density than theamorphous oxide semiconductor film.

<CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films having a pluralityof crystal parts. The crystal parts included in the CAAC-OS film eachhave c-axis alignment. In a plan TEM image, the area of the crystalparts included in the CAAC-OS film is greater than or equal to 2500 nm²,greater than or equal to 5 μm², or greater than or equal to 1000 μm².Further, in a cross-sectional TEM image, when the proportion of thecrystal parts is greater than or equal to 50%, greater than or equal to80%, or greater than or equal to 95% of the CAAC oxide film, the CAACoxide film is a thin film having physical properties similar to those ofa single crystal.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or the top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film. In this specification, aterm “parallel” indicates that the angle formed between two straightlines is greater than or equal to −10° and less than or equal to 10°,and accordingly also includes the case where the angle is greater thanor equal to −5° and less than or equal to 5°. In addition, a term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly includes the case where the angle is greater than orequal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(luminescent spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. When the CAAC-OS film is analyzed by anout-of-plane method, a peak appears frequently when the diffractionangle (2θ) is around 31°. This peak is derived from the (00x) plane (xis an integer) of the InGaZn oxide crystal, which indicates thatcrystals in the CAAC-OS film have c-axis alignment, and that the c-axesare aligned in a direction substantially perpendicular to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZn oxidecrystal. Here, analysis (φ scan) is performed under conditions where thesample is rotated around a normal vector of a sample surface as an axis(φ axis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZn oxide, six peaksappear. The six peaks are derived from crystal planes equivalent to the(110) plane. On the other hand, in the case of a CAAC-OS film, a peak isnot clearly observed even when φ scan is performed with 2θ fixed ataround 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of the top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Furthermore, when an impurity is added to the CAAC-OS film,the crystallinity in a region to which the impurity is added is changed,and the degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film is analyzed by an out-of-plane method, apeak of 2θ may also be observed at around 36°, in addition to the peakof 2θ at around 31°. The peak of 2θ at around 36° indicates that acrystal part having no c-axis alignment is included in part of theCAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θappear at around 31° and a peak of 2θ do not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “highly purified substantiallyintrinsic” state. A highly purified intrinsic or highly purifiedsubstantially intrinsic oxide semiconductor film has few carriergeneration sources, and thus can have a low carrier density. Thus, atransistor including the oxide semiconductor film rarely has negativethreshold voltage (is rarely normally on). The highly purified intrinsicor highly purified substantially intrinsic oxide semiconductor film hasa low density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Charges trapped by the carrier traps in the oxide semiconductor filmtake a long time to be released, and might behave like fixed charges.Thus, the transistor that includes the oxide semiconductor film havinghigh impurity concentration and a high density of defect states hasunstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Polycrystalline Oxide Semiconductor>

In an image obtained with a TEM, for example, crystal grains can befound in the polycrystalline oxide semiconductor film. In most cases,the size of a crystal grain in the polycrystalline oxide semiconductorfilm is greater than or equal to 2 nm and less than or equal to 300 nm,greater than or equal to 3 nm and less than or equal to 100 nm, orgreater than or equal to 5 nm and less than or equal to 50 nm in animage obtained with the TEM, for example. Moreover, in an image obtainedwith the TEM, a boundary between crystals can be found in thepolycrystalline oxide semiconductor film in some cases.

The polycrystalline oxide semiconductor film may include a plurality ofcrystal grains, and alignment of crystals may be different in theplurality of crystal grains. When a polycrystalline oxide semiconductorfilm is analyzed by an out-of-plane method with use of an XRD apparatus,a single peak or a plurality of peaks appear in some cases. For example,in the case of a polycrystalline IGZO film, a peak at 2θ of around 31°that shows alignment or plural peaks that show plural kinds of alignmentappear in some cases.

The polycrystalline oxide semiconductor film has high crystallinity andthus has high electron mobility in some cases. Accordingly, a transistorincluding the polycrystalline oxide semiconductor film has highfield-effect mobility. Note that there are cases in which an impurity issegregated at the grain boundary in the polycrystalline oxidesemiconductor film. Moreover, the grain boundary of the polycrystallineoxide semiconductor film serves as a defect state. Since the grainboundary of the polycrystalline oxide semiconductor film may serve as acarrier generation source or a trap state, a transistor including thepolycrystalline oxide semiconductor film has larger variation inelectrical characteristics and lower reliability than a transistorincluding a CAAC-OS film in some cases.

<Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a crystal grain cannot be found clearly in the nc-OS film in somecases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. However, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak that shows acrystal plane does not appear. Further, a halo pattern is shown in anelectron diffraction pattern (also referred to as a selected-areaelectron diffraction pattern) of the nc-OS film obtained by using anelectron beam having a probe diameter (e.g., larger than or equal to 50nm) larger than the diameter of a crystal part. Meanwhile, spots areshown in a nanobeam electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to, orsmaller than or equal to the diameter of a crystal part. Furthermore, ina nanobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

FIG. 20 shows an example of nanobeam electron diffraction performed on asample including an nc-OS film. The measurement position is changed.Here, the sample is cut in the direction perpendicular to a surfacewhere an nc-OS film is formed and the thickness thereof is reduced to beless than or equal to 10 nm. Further, an electron beam with a diameterof 1 nm enters from the direction perpendicular to the cut surface ofthe sample. FIG. 20 shows that, when a nanobeam electron diffraction isperformed on the sample including the nc-OS film, a diffraction patternexhibiting a crystal plane is obtained, but orientation along a crystalplane in a particular direction is not observed.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 8

In the method for manufacturing any of the transistors described in theabove embodiments, after the conductive films 21 a and 21 b serving as apair of electrodes are formed, the oxide semiconductor film 19 a may beexposed to plasma generated in an oxidizing atmosphere, so that oxygenmay be supplied to the oxide semiconductor film 19 a. Atmospheres ofoxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like canbe given as examples of oxidizing atmospheres. Further, in the plasmatreatment, the oxide semiconductor film 19 a is preferably exposed toplasma generated with no bias applied to the substrate 11 side.Consequently, the oxide semiconductor film 19 a can be supplied withoxygen without being damaged; accordingly, the number of oxygenvacancies in the oxide semiconductor film 19 a can be reduced. Moreover,impurities, e.g., halogen such as fluorine or chlorine remaining on asurface of the oxide semiconductor film 19 a due to the etchingtreatment can be removed. The plasma treatment is preferably performedwhile heating is performed at a temperature higher than or equal to 300°C. Oxygen in the plasma is bonded to hydrogen contained in the oxidesemiconductor film 19 a to form water. Since the substrate is heated,the water is released from the oxide semiconductor film 19 a.Consequently, the amount of hydrogen and water in the oxidesemiconductor film 19 a can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 9

Although the oxide semiconductor films that are described in the aboveembodiments can be formed by a sputtering method, such films may beformed by another method, e.g., a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ALD) method may be employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on a surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the metal film, the oxide semiconductorfilm, and the inorganic insulating film that are described in the aboveembodiment can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc are used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas that is obtained bybubbling with an inert gas such as Ar may be used instead of an O₃ gas,it is preferable to use an O₃ gas, which does not contain H. Further,instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Instead of an In(CH₃)₃ gas,an In(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may be used.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 10

A semiconductor device (also referred to as a display device) having adisplay function can be manufactured using the transistor examples ofwhich are shown in the above embodiments. Moreover, some or all of thedriver circuits that include the transistor can be formed over asubstrate where the pixel portion is formed, whereby a system-on-panelcan be obtained. In this embodiment, an example of a display deviceusing the transistor examples of which are shown in the aboveembodiments is described with reference to FIGS. 21A to 21C and FIGS.22A and 22B. FIGS. 22A and 22B are cross-sectional views illustratingcross-sectional structures taken along dashed-dotted line M-N in FIG.21B.

In FIG. 21A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with a second substrate 906. In FIG. 21A, a signal line drivercircuit 903 and a scan line driver circuit 904 each are formed using asingle crystal semiconductor or a polycrystalline semiconductor over asubstrate prepared separately, and mounted in a region different fromthe region surrounded by the sealant 905 over the first substrate 901.Further, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from a flexible printed circuit (FPC) 918.

In FIGS. 21B and 21C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 that are providedover the first substrate 901. The second substrate 906 is provided overthe pixel portion 902 and the scan line driver circuit 904. Thus, thepixel portion 902 and the scan line driver circuit 904 are sealedtogether with a display element by the first substrate 901, the sealant905, and the second substrate 906. In FIGS. 21B and 21C, a signal linedriver circuit 903 that is formed using a single crystal semiconductoror a polycrystalline semiconductor over a substrate separately preparedis mounted in a region different from the region surrounded by thesealant 905 over the first substrate 901. In FIGS. 21B and 21C, varioussignals and potentials are supplied to the signal line driver circuit903, the scan line driver circuit 904, and the pixel portion 902 from anFPC 918.

Although FIGS. 21B and 21C each show an example in which the signal linedriver circuit 903 is formed separately and mounted on the firstsubstrate 901, one embodiment of the present invention is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 21A shows an example in which the signal line drivercircuit 903 and the scan line driver circuit 904 are mounted by a COGmethod. FIG. 21B shows an example in which the signal line drivercircuit 903 is mounted by a COG method. FIG. 21C shows an example inwhich the signal line driver circuit 903 is mounted by a TAB method.

The display device includes in its category a panel in which a displayelement is sealed and a module in which an IC including a controller orthe like is mounted on the panel.

A display device in this specification refers to an image display deviceor a display device. Further, the display device also includes thefollowing modules in its category: a module to which a connector such asan FPC or a TCP is attached; a module having a TCP at the tip of which aprinted wiring board is provided; and a module in which an integratedcircuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors and any of thetransistors that are described in the above embodiments can be used. Anyof the transistors described in the above embodiments can be applied toa buffer circuit included in the scan line driver circuit.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. A light emitting element includes, in its scope,an element whose luminance is controlled by current or voltage, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Further, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used. FIG. 22A illustrates an example of a liquid crystal displaydevice using a liquid crystal element as the display element and FIG.22B illustrates an example of a light-emitting display device using alight-emitting element as the display element.

As illustrated in FIGS. 22A and 22B, the display device includes aconnection terminal electrode 915 and a terminal electrode 916. Theconnection terminal electrode 915 and the terminal electrode 916 areelectrically connected to a terminal included in the FPC 918 through ananisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930, and the terminal electrode 916is formed using the same conductive film as a pair of electrodes in eachof a transistor 910 and a transistor 911.

Each of the pixel portion 902 and the scan line driver circuit 904 thatare provided over the first substrate 901 includes a plurality oftransistors. FIGS. 22A and 22B illustrate the transistor 910 included inthe pixel portion 902 and the transistor 911 included in the scan linedriver circuit 904. In FIG. 22A, an oxide insulating film 924 isprovided over each of the transistors 910 and 911, and an oxygen barrierfilm 927 is provided over the oxide insulating film 924, and in FIG.22B, a planarization film 921 is further provided over the oxygenbarrier film 927.

In this embodiment, any of the transistors described in the aboveembodiments can be used as the transistors 910 and 911 as appropriate.By using any of the transistors described in the above embodiments asthe transistors 910 and 911, a display device with high image qualitycan be fabricated.

Moreover, FIG. 22B shows an example in which a conductive film 917 isprovided over the oxygen barrier film 927 so as to overlap a channelregion of an oxide semiconductor film 926 of the transistor 911 for thedriver circuit. In this embodiment, the conductive film 917 is formedusing the conductive film that is used as the first electrode 930. Byproviding the conductive film 917 so as to overlap the channel region ofthe oxide semiconductor film, the amount of change in the thresholdvoltage of the transistor 911 between before and after a BT stress testcan be further reduced. The conductive film 917 may have the samepotential as or a potential different from that of the gate electrode ofthe transistor 911, and the conductive film 917 can serve as a secondgate electrode. The potential of the conductive film 917 may be GND, 0V, in a floating state, or the same potential or substantially the samepotential as the minimum potential (Vss; for example, the potential ofthe source electrode in the case where the potential of the sourceelectrode is a reference potential) of the driver circuit.

In addition, the conductive film 917 has a function of blocking anexternal electric field. In other words, the conductive film 917 has afunction of preventing an external electric field (particularly, afunction of preventing static electricity) from affecting the inside (acircuit portion including the transistor). Such a blocking function ofthe conductive film 917 can prevent change in electrical characteristicsof the transistor due to the influence of an external electric fieldsuch as static electricity. The conductive film 917 can be used for anyof the transistors described in the above embodiments.

In the display panel, the transistor 910 included in the pixel portion902 is electrically connected to a display element. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beused.

In FIG. 22A, a liquid crystal element 913 that is a display elementincludes the first electrode 930, a second electrode 931, and a liquidcrystal layer 908. Note that an insulating film 932 and an insulatingfilm 933 that serve as alignment films are provided so that the liquidcrystal layer 908 is provided therebetween. The second electrode 931 isprovided on the second substrate 906 side. The second electrode 931overlaps the first electrode 930 with the liquid crystal layer 908provided therebetween.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may be used.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is raised. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. The liquid crystal composition thatincludes a liquid crystal showing a blue phase and a chiral material hasa short response time of 1 msec or less, and has optical isotropy, whichmakes the alignment process unneeded and viewing angle dependence small.In addition, since an alignment film does not need to be provided andrubbing treatment is unnecessary, electrostatic discharge damage causedby the rubbing treatment can be prevented and defects and damage of theliquid crystal display device in the manufacturing process can bereduced. Thus, the productivity of the liquid crystal display device canbe increased.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used.

Further, the transistor including an oxide semiconductor film used inthe above embodiments has excellent switching characteristics.Furthermore, relatively high field-effect mobility is obtained, whichenables high-speed operation. Consequently, when the above transistor isused in a pixel portion of a semiconductor device having a displayfunction, high-quality images can be obtained. Since a driver circuitportion and the pixel portion can be formed over one substrate with theuse of the above transistor, the number of components of thesemiconductor device can be reduced.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charges can be held for apredetermined period. By using the transistor including thehighly-purified oxide semiconductor film, it is enough to provide astorage capacitor having a capacitance that is ⅓ or less, or ⅕ or lessof a liquid crystal capacitance of each pixel; therefore, the apertureratio of a pixel can be increased.

In the display device, a black matrix (a light-blocking film), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be used. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite), or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween respective dots of color elements. One embodiment of the presentinvention is not limited to the application to a display device forcolor display but can also be applied to a display device for monochromedisplay.

In FIG. 22B, a light-emitting element 963 that is a display element iselectrically connected to the transistor 910 provided in the pixelportion 902. Note that although the structure of the light-emittingelement 963 is a stacked-layer structure of the first electrode 930, alight-emitting layer 961, and the second electrode 931, the structure isnot limited thereto. The structure of the light-emitting element 963 canbe changed as appropriate depending on the direction in which light isextracted from the light-emitting element 963, or the like.

A partition wall 960 can be formed using an organic insulating materialor an inorganic insulating material. It is particularly preferable thatthe partition wall 960 be formed using a photosensitive resin materialto have an opening over the first electrode 930 so that a sidewall ofthe opening has an inclined surface with a continuous curvature.

The light-emitting layer 961 may be formed to have a single-layerstructure or a stacked-layer structure including a plurality of layers.

A protective film may be formed over the second electrode 931 and thepartition wall 960 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering the light-emitting element963. As the protective film, a silicon nitride film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum nitride film, analuminum oxynitride film, an aluminum nitride oxide film, a DLC film, orthe like can be formed. In addition, in a space that is sealed with thefirst substrate 901, the second substrate 906, and a sealant 936, afiller 964 is provided and sealed. It is preferable that, in thismanner, the light-emitting element be packaged (sealed) with aprotective film (such as a laminate film or an ultraviolet curable resinfilm) or a cover material with high air-tightness and littledegasification so that the panel is not exposed to the outside air.

As the sealant 936, an organic resin such as a thermosetting resin or aphotocurable resin, fritted glass including low-melting glass, or thelike can be used. The fritted glass is preferable because of its highbarrier property against impurities such as water and oxygen. Further,in the case where the fitted glass is used as the sealant 936, asillustrated in FIG. 22B, the fritted glass is provided over the oxideinsulating film 924, whereby adhesion of the oxide insulating film 924to the fritted glass becomes high, which is preferable.

As the filler 964, as well as an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used:polyvinyl chloride (PVC), an acrylic resin, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), ethylene vinyl acetate (EVA),or the like can be used. For example, nitrogen is used for the filler.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate for a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

The first electrode and the second electrode (each of which may becalled a pixel electrode, a common electrode, a counter electrode, orthe like) for applying voltage to the display element may havelight-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930 and the second electrode 931 can be formed usinga light-transmitting conductive material such as indium oxide includingtungsten oxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,ITO, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Alternatively, the first electrode 930 and the second electrode 931 canbe formed using one or more materials selected from metals such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); an alloy of any of these metals; and a nitride of any ofthese metals.

The first electrode 930 and the second electrode 931 can be formed usinga conductive composition including a conductive macromolecule (alsoreferred to as a conductive polymer). As the conductive high molecule,what is called a π-electron conjugated conductive polymer can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, a copolymer of two or more of aniline, pyrrole, andthiophene, and the like can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

As described above, by using any of the transistors described in theabove embodiments, a highly reliable semiconductor device having adisplay function can be provided.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no.2013-103716 filed with Japan Patent Office on May 16, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstinsulating film over a substrate; a first oxide insulating film over thefirst insulating film; an oxide semiconductor film including a channelformation region over and in contact with the first oxide insulatingfilm; a second oxide insulating film over and in contact with the oxidesemiconductor film; and a second insulating film over the second oxideinsulating film, wherein the first insulating film and the secondinsulating film are in physical contact with each other, and wherein theoxide semiconductor film, the second oxide insulating film, and a partof the first oxide insulating film are provided on an inner side of thefirst insulating film and the second insulating film.
 2. Thesemiconductor device according to claim 1, further comprising: a firstgate electrode between the substrate and the first insulating film; anda second gate electrode over the second insulating film, wherein thefirst gate electrode and the second gate electrode are in contact witheach other in an opening in the first insulating film and the secondinsulating film.
 3. The semiconductor device according to claim 2,wherein a side surface of the oxide semiconductor film faces the secondgate electrode with the second oxide insulating film and the secondinsulating film provided therebetween.
 4. The semiconductor deviceaccording to claim 1, wherein each of the first insulating film and thesecond insulating film is one selected from a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,a gallium oxide film, a gallium oxynitride film, an yttrium oxide film,an yttrium oxynitride film, a hafnium oxide film, and a hafniumoxynitride film.
 5. The semiconductor device according to claim 1,wherein the second oxide insulating film includes an oxide filmcontaining more oxygen than that in a stoichiometric composition,wherein a released amount of oxygen atoms from the oxide film in thermaldesorption spectroscopy analysis is greater than or equal to 1.0×10¹⁸atoms/cm³, and wherein a surface temperature of the oxide film in thethermal desorption spectroscopy is higher than or equal to 100° C. andlower than or equal to 700° C.
 6. A semiconductor device comprising: atransistor comprising: a first insulating film over a substrate; a firstoxide insulating film over the first insulating film; an oxidesemiconductor film over and in contact with the first oxide insulatingfilm; source and drain electrodes in contact with the oxidesemiconductor film; a second oxide insulating film over and in contactwith the oxide semiconductor film; and a second insulating film over thesecond oxide insulating film; a pixel electrode over the secondinsulating film; and a capacitor comprising; a film having conductivityover the first oxide insulating film; the second insulating film overand in contact with the film having conductivity; and the pixelelectrode, wherein the first insulating film and the second insulatingfilm are in physical contact with each other, and wherein the pixelelectrode is in contact with one of the source and drain electrodes inan opening in the second insulating film.
 7. The semiconductor deviceaccording to claim 6, wherein the film having conductivity is a metaloxide film including a metal element contained in the oxidesemiconductor film and an impurity.
 8. The semiconductor deviceaccording to claim 7, wherein the impurity is hydrogen.
 9. Thesemiconductor device according to claim 6, further comprising a gateelectrode over the second insulating film, wherein the gate electrodeand the pixel electrode include a same material.
 10. The semiconductordevice according to claim 9, wherein a side surface of the oxidesemiconductor film faces the gate electrode with the second oxideinsulating film and the second insulating film provided therebetween.11. The semiconductor device according to claim 6, wherein each of thefirst insulating film and the second insulating film is one selectedfrom a silicon nitride film, a silicon nitride oxide film, an aluminumnitride film, an aluminum nitride oxide film, an aluminum oxide film, analuminum oxynitride film, a gallium oxide film, a gallium oxynitridefilm, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxidefilm, and a hafnium oxynitride film.
 12. The semiconductor deviceaccording to claim 6, wherein the second oxide insulating film includesan oxide film containing more oxygen than that in a stoichiometriccomposition, wherein a released amount of oxygen atoms from the oxidefilm in thermal desorption spectroscopy analysis is greater than orequal to 1.0×10¹⁸ atoms/cm³, and wherein a surface temperature of theoxide film in the thermal desorption spectroscopy is higher than orequal to 100° C. and lower than or equal to 700° C.
 13. A semiconductordevice comprising: a transistor comprising: a first gate electrode overa substrate; a first insulating film over the first gate electrode; afirst oxide insulating film over the first insulating film; an oxidesemiconductor film over and in contact with the first oxide insulatingfilm; source and drain electrodes in contact with the oxidesemiconductor film; a second oxide insulating film over and in contactwith the oxide semiconductor film; a second insulating film over thesecond oxide insulating film; and a second gate electrode over thesecond insulating film; a pixel electrode over the second insulatingfilm; and a capacitor comprising; a film having conductivity over thefirst oxide insulating film; the second insulating film over and incontact with the film having conductivity; and the pixel electrode,wherein the first insulating film and the second insulating film are inphysical contact with each other, wherein the first gate electrode andthe second gate electrode overlap each other with the oxidesemiconductor film provided therebetween, and wherein the pixelelectrode is in contact with one of the source and drain electrodes in afirst opening in the second insulating film.
 14. The semiconductordevice according to claim 13, wherein the first gate electrode and thesecond gate electrode are in contact with each other in a second openingin the first insulating film and the second insulating film.
 15. Thesemiconductor device according to claim 13, wherein the film havingconductivity is a metal oxide film including a metal element containedin the oxide semiconductor film and an impurity.
 16. The semiconductordevice according to claim 15, wherein the impurity is hydrogen.
 17. Thesemiconductor device according to claim 13, wherein the second gateelectrode and the pixel electrode include a same material.
 18. Thesemiconductor device according to claim 13, wherein a side surface ofthe oxide semiconductor film faces the second gate electrode with thesecond oxide insulating film and the second insulating film providedtherebetween.
 19. The semiconductor device according to claim 13,wherein each of the first insulating film and the second insulating filmis one selected from a silicon nitride film, a silicon nitride oxidefilm, an aluminum nitride film, an aluminum nitride oxide film, analuminum oxide film, an aluminum oxynitride film, a gallium oxide film,a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitridefilm, a hafnium oxide film, and a hafnium oxynitride film.
 20. Thesemiconductor device according to claim 13, wherein the second oxideinsulating film includes an oxide film containing more oxygen than thatin a stoichiometric composition, wherein a released amount of oxygenatoms from the oxide film in thermal desorption spectroscopy analysis isgreater than or equal to 1.0×10¹⁸ atoms/cm³, and wherein a surfacetemperature of the oxide film in the thermal desorption spectroscopy ishigher than or equal to 100° C. and lower than or equal to 700° C.